I use an iPhone X.
If I’m on WIFI, it runs smoothly, no problems. If I’m not on WiFi during start up it will say “unable to login,” with the options “retry” or “try a different login.” Neither of which do anything.
If I’m on WiFi and turn it off mid game it will stop working and give me a red banner at the top saying “no internet connection (10)”
I’ve tried redownloading the app, I’ve tried logging in different ways. I’ve checked to see if the app permissions were all good to go in the iPhone and PG app settings (so far).
Very frustrating bug, it’s not really Pokémon go if I can’t play it on the go.