While it’s nice to have REM to verify (scientifically by measurement) whether your hearing aids setup is on target or not, it’s not the end of the world if you don’t get to do REM to verify.
The alternative is to “verify by ear”. Of course this is the less desired alternative because it’s subjective by having your brain deciding whether you hear well enough yet, and usually it would be the longer route to get to where you’re satisfied with the hearing aids.
For example, if your hearing aids setup underperforms to target across all frequency range, and no REM is available and done, then your brain will perceive that you’re not hearing loud enough. So you complain to your audi and they increase the overall gain for you a little at a time until you’re happy. So this would be a much more crude (and time consuming) way to make the (almost) equivalent of the up-front REM adjustment. But as long as you get there eventually and are happy with your hearing aids’ performance after several visits to the audi for adjustments, in the end, you still end up achieving the same goal → you can hear better.
Of course if your hearing aids’ setup underperforms only in certain frequency ranges and not across the board, then it’d be much more complicated to try to get it right than a simple across-the-board volume increase like in the simplified example above. But the principle is still the same → you keep having your audi do (more complicated) adjustments until you’re happy.
I’m not saying this as an excuse for not having REM done when possible. I’m just saying that if REM is not possible at all, it’s still not the end of the world as long as you eventually get to the point where you are satisfied with your hearing aids’ performance. It may take several more visits and adjustments to achieve this compared to if REM is done, but as long as the audi is competent to adjust until you’re happy, then it may be a longer journey to get there, but eventually hopefully you get there.