I don’t have the Starkey Halo 2, I have the Oticon OPN. But I have a similar issue of losing connection between my iPhone and one of the OPN from time to time. On the OPN you simply set the iPhone to forget the device (the OPN), then you open/close the battery door of the OPN to reset it, and this usually reconnects the iphone with the OPN. Some people can just turn the iPhone Bluetooth mode off then on again and this reconnects the OPN for them, but I haven’t had success doing so this way.
I would assume that you can do the same on your Halo 2 (forget device on iPhone and reset the Halo 2) to reconnect. All these settings is under the iPhone -> Settings -> General -> Accessibility -> Hearing Devices -> Devices (at the top which shows your Halo 2 under your name)