downloaded latest compatible version of app, not working!

I downloaded the latest compatible version of a banking app to my iphone 6 (up to date v.12.5.6). I understand that the latest version isn't compatible with my phone, but the Apple store asked me if I wanted to download the latest version compatible with my iphone 6, which I did. However after I open the app, I cannot do anything. I called my bank's tech support and they said it should work. So, who do I address to fix the issue (or just take that version off the Apple store?) Same thing with Google Voice app. I can download and open the latest version compatible with my iphone 6, but then get a message telling me the app is out of date. If I just click "OK" the app closes, if I click on "update" I get inot a "loop" where I can only download the latest version compatible with my iphone 6... So going nowhere with these "latest compatible versions. Newer iphones are still very expensive, even on back markets. I think it's cruel to force people to buy a new phone all the time. No wonder so many users end up buying non-Apple smartphones!