I have ReSound Cala hearing aids (Costco, 2016) which are related to the Linx2 models that were sold through audiologists. They are compatible with the ReSound Smart app (but not Smart 3D), and I have been using that on an MFi through various updates since 2016. The app was always somewhat balky but lately has gotten quite unstable in use with my iPhone 6S. I have had to delete and reinstall numerous times because it either keeps going into automatic iPhone mode, or splits on its own into L and R channels and mutes the L, or it constantly reboots, or it mutes when I press the home button or side switch on the phone, to navigate to another app, etc. Perhaps the most maddening change is a little notification I get when a battery is running low. Not only will the notice not go away but it freezes the screen and the entire app–even when I replace the batteries. It was doing some of this before I upgraded the iOS from 13.7 to 14.3, but now it is worse. (Some say the iPhone 6 is not compatible with iOS 14, but it loaded the upgrade and works pretty normally otherwise.) Has anyone else had similar problems with the Smart app? I may soon be upgrading my phone and hoping that will solve the app problems, but I suspect that ReSound needs to fix the bugs in the current iteration of the Smart app to be more compatible with the new iOS. I keep getting worthless emails from the developer when I leave a bad review, but notice no new update; the latest is over a month old.
I can state from personal experience that the iPhone 6 (not 6 S) can NOT be updated to 14.x. The most current version of iOS for the 6 is 12.5 and that is recent. The 6 won’t run the current version of the Oticon ON app for iOS - it now requires 13 or later. Other apps I had won’t run now on the (ancient) 6 as they also require a newer version of iOS
As you found, your iPhone 6 S is compatible with the more recent versions of iOS. So that should not be the problem - it is may be that the app doesn’t deal with the new version of iOS or used some “undocumented” feature of the earlier iOS that is no longer supported.
As the battery in my 6 just quit, I updated to an iPhone 12 mini and with 14.3, things work well (including the ON app) with my Oticon OPN S 1 aids. I have no idea if that would be true for ReSound apps/aids - for me the phone upgrade is useful although I’m having some issues without a home button on the 12 
Good luck.
My daughter’s old, tired, 6S was giving up the ghost, so I upgraded her to the iPhone SE 2020. Right now, it’s a very cost-effective option. She really liked her 6S and the SE has the identical functionality, with very few differences in operation. It does have some significant improvements like a more powerful processor and better camera.
She did a cloud backup on her old phone, so when the new one was activated, it restored everything just like it was on her old phone. She says it’s exactly like her 6S but much snappier.
Depending on your carrier, the SE may only cost you $10 a month more.
I have an iPhone 6S that I just inherited from my wife to see if I wanted to switch from Android to Apple. It has been upgraded to iOS 14.3 and has a battery with 99% of its capacity left as Apple replaced its battery just before the wife upgraded to a newer iPhone.
My iPhone 6S works great with the ReSound Smart 3D app. My Android phone is a 3-years-newer 2018 Galaxy Note 8 running the Microsoft Launcher and even though the Launcher interface and most apps are very snappy on my Note 8, the behavior of the Smart 3D app on the iPhone 6S is great, very snappy compared to the Galaxy Note 8. The reason for switching to the iPhone 6S was to test how I like the Apple Watch 6 controlling my HA’s through the Smart 3D app running on the Apple Watch but linked to my iPhone. That works great, too. So I would say, based on my experience, that your problem might not be that the iPhone 6S in general does not work well with the Smart 3D app or iOS 14.3 but rather, perhaps, your particular individual iPhone 6S may be showing its age. My iPhone 6S has sat unused for several years before I asked my wife to let me have it to try out an Apple Watch and she gave the phone a lot of TLC when she used it. The other critical difference is I am running rechargeable Quattro 961’s. So perhaps the ReSound app does have a problem with disposable zinc-air batteries but I don’t have any problem with the iPhone 6S monitoring the charge state of my rechargeables. I mentioned in another thread that there is an iOS app called Battery Life for both the iPhone and the Apple Watch. It’s intended to monitor the charge state of the iPhone and Apple Watch batteries (one from the other) but somehow it also picks up the charge state of my Quattro Li-ion batteries from one or both HA’s in a somewhat flaky way. It might be interesting if someone tried the Battery Life app with ReSound HA’s using disposable batteries and reported what it does. The app is offered on the “freemium” model. You get some basics for free but if you want the whole enchilada, it costs something like $4. Battery Life on the App Store (apple.com)
Good Luck with the Smart 3D app. There might be a problem with disposable batteries but otherwise I can say the app, the iPhone 6S, and iOS 14.3 work great together and also great with an Apple Watch 6 running WatchOS 7.2 although as others have noted, you can only run the Smart 3D app thru a complication on a more modern Apple Watch face like Infograph by creating an Apple Shortcut to launch the 3D app: Apple Watch
Edit_Update: Something else I do that might make a difference in the general behavior of one’s iPhone. In Settings under Battery, being the battery-conserving-monger that I am, I always run my iPhone in Low-Power Mode to keep ~all apps that would like to try from running in the background. YMMV, but no text, e-mail, Whats App msg, etc., is so important that I have to be notified right away (and I still seem to get notifications anyway from certain apps like Ring and Outlook). So if you have a lot of apps that have requested permission to do something in the background, that could be a difference in performance between your iPhone 6S and mine. I tell ~all apps that they’re only allowed to use my location when I’m running the app, etc.
As I mentioned in my original post, I have Resound Calas that will not operate the Smart 3D app, only the older Smart app. I could be wrong, but since the 3D is the most current and in-demand app, I suspect Resound’s softwear engineers upgrade it more frequently and more carefully than the Smart app I must use. Anyway, by deleting and reinstalling the Smart app numerous times (also unpairing and repairing the HAs) and also tweaking various settings on my iPhone 6S I have managed to establish a workable relationship with the app, pending purchase of a newer iPhone (my 6S only has 75 percent battery life and needs a frequent umbilical cord to a charger, despite various adjustments to minimize battery drainage). The Smart app now goes into streaming mode when I connect to the TV streamer, doesn’t reboot constantly, and only rarely defaults to iPhone mode (but still does so if I am browsing the internet while watching TV through the streamer). One new thing that’s happening in 14.3 is that hitting the home button or the side switch on the 6S knocks it out of streaming mode, at least temporarily. The other annoying behavior of the current app update is the low battery warning, which appears on my phone screen, locks the app, and won’t go away until and sometimes even after I change the batteries. So far, I have had no problems with telephone calls, as long as the batteries have some charge left. I am still not hearing anything from other Smart app (not 3D) users and would be curious to know of their experiences.
Yeah, sorry I blithely ignored your statement and the ReSound compatibility web page information that anything earlier than the LiNX 3D is only compatible with the ReSound Smart app.
But the poor charge state of your iPhone may explain your issues in part (only 75% capacity left, you say). I recently had an experience with a Samsung smartwatch in which the battery was failing. The watch could no longer execute many of its app functions properly. Bought a replacement battery kit on Amazon, the watch survived the “open heart” surgery and BINGO!, everything was back to working as it should with a new battery. If you remember Apple’s “batterygate” a few years back, they were cutting back on the performance of iPhones with older batteries (without telling anyone) to prevent the whole OS from going down in flames if iPhone apps demanded more power at once than an older battery could provide. Now power management in iPhones is user-configurable. You might check whether you’ve put limits on sudden battery drain or not and try turning the option on or off (the opposite of whatever way you have it now) to see if it makes a difference. But Apple has considered 80% capacity the point at which you should replace the battery (or the iPhone!) - I wouldn’t be surprised if phone voltage problems or current drain issues are contributing to your problems. With a less than ideal phone battery, allowing a number of apps to run in the background could also contribute to performance issues. With a bad smartwatch battery, I could get along with many apps. Only certain things, like trying to make a payment at a credit card terminal caused the watch to hang or go down in flames. Maybe your situation is somewhat similar?
From looking at battery settings on my phone it appears Apple has built in various charge-saving responses that trigger automatically when the battery capacity is diminished. The low-power setting is described as “temporary”–until the phone can be recharged. The app problems seem to be about the same even when the phone is connected to a charger, and what has been happening does not to me seem likely to be a matter of peak-power capability, though of course I could be wrong. I’m more inclined to think there’s some instability between the app and the newest iOS, and additionally it may be that because this old iPhone has a home button and the newer ones do not, there could be some glitch triggered every time I have to hit that button to change a setting or an app in use. Oddly enough, just now I had a HA battery get low, received the usual notice on the app screen, hit OK, and this time the notice went away and the app screen did not lock. I wonder if ReSound tweaked the app without announcing a full upgrade. Meanwhile, we will be looking at new phones, probably the 12, and hopefully making that switch in the next week or so. Thanks for your thoughtful comments!
Seems the new SE2020 has a home button. I was thinking about getting it recently but all the complaints about Apple Bluetooth lately steered me away from the iPhone.
The low battery sure might be the issue. Apple might think Bluetooth connections as non-essential.
I was thinking about joking in another thread where I mentioned the value of perseverance is that one thing that I learned in a Microsoft AI course or two is that even not-so-smart bots can eventually conquer a confounding maze through relentless effort and a probabilistic memory of the relative long-term value of going in directions A, B, or C from point Z. I think I was able to solve some of the course Python programming challenges by applying that philosophy to sometimes uninspired programming on my part - and in my “helpful comments” here, I may just be reapplying such semi-methodical wandering in hopes of finding the finish line somewhere, somehow, by hook or by crook! 
No one has chimed in to say how the ReSound Smart app works with an iPhone SE2020 and its home button.
I was not aware of Apple having more Bluetooth problems than Android or other devices. At any rate my HAs are described as paired, and if they weren’t I wouldn’t be able to perform any functions at all with the Smart app.
Just replying to your comment.
The forum is full of iOS 14 update problems.
Just trying to help.
Good luck
Just commenting on a few points:
The Resound app doesn’t report on battery levels for LINX Quattro HAs that use zinc-air batteries. All it does is remember in the app if one or other HA has beeped to indicate its need for new batteries. I guess this is different to the rechargeable because whereas you can deduce the state of charge of a lithium battery from its output voltage, you can’t do this with a zinc-air battery because the output voltage remains more or less constant until it crashes upon exhaustion.
Whether the phone uses a home button or gestures and face recognition makes no difference to the behaviour of the Smart 3D app.
You can customise the behaviour of the side button in an iPhone if you want to.
Running a phone in low power mode is a good way to preserve limited phone charge, but it will have zero effect on the behaviour of the Resound app, except that it might stop the Resound app working in the background. So low power mode is to be avoided if you have problems with the phone and HAs talking to each other.
And recent issues with IOS 14 and HAs have all been limited to the iPhone 12. This wasn’t a problem of the IOS 14 itself, but because Apple had inadequately tested their new Bluetooth configuration in the iPhone 12, consequent on using a different Bluetooth chip. They mostly fixed their general iPhone 12 Bluetooth issues with IOS 14.2 and fixed the HA Bluetooth issues with IOS 14.2.1.
All current iphones should work properly with Resound hearing aids listed on Resound’s website. But you have to keep the firmware of the aid and the phone up to date and if the battery in the phone is down below 80% then it’s time to have it changed.
I think most of my commentary, at least, is directed towards the idea that there might be something wrong with the OP’s iPhone, since mine works great under the same circumstances, and that much of what’s wrong with the iPhone might be that the iPhone’s battery is no longer up to snuff. The OP does have the problem that when his disposable zinc-air batteries are about exhausted the notification to the Smart app on his iPhone creates a notification that won’t go away (or something like that). So I’d say the only curious and unusual suggestion is that I’ve observed the iOS app Battery Life can somehow pick up (in my case) the wireless packets my ReSound HA’s send to the Smart 3D app about their charge state, which seems highly unusual and somewhat against Apple’s vaunted privacy policies. I was just curious, since the OP’s HA’s clearly send a wireless depleted battery notification to his iPhone and to the ReSound Smart app on the phone, whether the Battery Life app somehow picks up that notification, too. Probably not. But if you’d asked me a priori, would the Battery Life app (designed for iPhone and Apple Watch battery charge state detection) pick up the continuous charge state of my ReSound Quattro’s, I would have said, “Probably not! Why would an iOS/WatchOS battery charge state app be able to read ReSound HA charge state?” I like to think and explore outside the box.
The other thing that is not conclusive is that although the ReSound Smart app does not report the zinc-air battery charge state to the user, the HA firmware is probably monitoring the charge state of that type of battery in a rather constant manner. The reason for supposing that is ESPECIALLY because zinc-air batteries come crashing down in voltage level so suddenly as they approach depletion. You want to give the user the maximum warning possible to know of an impending power loss and be able to change the batteries well before that happens. So whether or not, the Smart app reports on a voltage level, I’d bet the HA’s are rather continuously checking. Did ReSound invent a whole different way for HA’s with disposable batteries vs. Li-ion batteries to report to the mothership (the app on the smartphone)? It would be interesting, if the Battery Life app has an inadvertent way of intercepting diplomatic messages from HA’s to smartphone, whether ReSound zinc-air battery-powered HA’s actually show up in the Battery Life app and whether anything about their charge state, at least the depletion notification, is ever registered there. The Battery Life app tells me whether it’s detecting the left or right Quattro and it gives me a fixed identifier for each HA. I think the naming and identification comes through the iOS Hearing Devices applet in the Accessibility settings of iOS. So what’s going on here might be beyond ReSound programming. It kinda seems like the MFi standards are the foundation of what’s going on and the ReSound app, perhaps, is just built on top of Apple’s MFi piping, so to speak. So if that’s what’s going on, my TL;DR conclusion is that the Battery Life app might also be able to tune in on what other brands of rechargeable Li-ion-based MFi HA’s are telling the mothership and also, I’d be curious if anything about zinc-air powered MFi HA’s shows up in the MFi piping as reported by the Battery Life app, too. If there’s already a MFi superhighway laid down for communication, why (re)invent another communication method thru iOS for each MFi HA brand and model, rechargeable or not?
The notification in the Smart 3D app is just that a red warning mark appears against status in the home screen and if you select status you get an explanatory message. If you want to get rid of it, all you have to do is change the battery.
But what is this Battery Life app? Ah so this is a third party app? So I installed it and it is telling me that my hearing aids are at 90% full charge (they are rechargeable). If the app gives notifications that won’t go away then deleting the app and relying on Apple and Resound seems a good plan.
Sorry for all the quotes! Saves retyping!
Actually, the basic reporting of the Battery Life app that I’m referencing here and am suggesting is somehow able to intercept MFi/iOS battery state notifications is part of the FREE version of the app. So anyone curious could try the free Battery Life app with their brand of MFi HA’s and report back (maybe in a new thread?!). If you have more than one iOS device or an Apple Watch, the Battery Life app allows you to check the charge state of all such devices from one device. If you constantly use the app to check charge state, you’re obviously going to use up a bit of the charge on the device you’re using it from because of screen time, etc., so it saves host battery to use the app judiciously. If you don’t suspend Background App Refresh in iOS, you can also use the app to send you high and low charge level notifications for each iOS/WatchOS device - but that feature only comes as part of the paid $4 version of the app.
I don’t think there are any obvious bugs in the Smart 3D app currently, at least not that I have noticed with my iPhone 12 Pro. The app was last updated on 23rd December incidentally.
I have had problems for years with phones and ha. The Plantronics Bluetooth works better than hearing aids. Unfortunately the plantronics does not work with a landline. I wish ha manufacturers would make one.