You should just get a VPN if you don't trust public WiFi. There are so many places with weak T-Mobile signals but strong public WiFi like Disneyland. T-Mobile is not very strong there and in some indoor lines, you get no signal at all, but Disneyland WiFi is strong. Last thing you want is wait in a 90 min line and can't load anything on your phone LOL. Also when you try to load data with a weak cell signal, not only loading is slow, the worst part is that it really drains your battery fast.

Also for T-Mobile coverage, if you found yourself seems to be getting low bar 5G on your phone, change preferred network type to LTE from 5G. Most places with weak 5G get strong LTE, and unless you are watching 4K Netflix or something like that, you won't even noticed the difference in speed. Also LTE uses less battery as well.