The only way I could ever get anything approaching reliable cell tower connection was by setting the phone to connect by WCDMA only.
T-Mo has now shut that service down in the US, so that pretty much bricks the device as a phone now.
Mine always had decent WiFi connection, so maybe you could get VoWiFi working when you have a hotspot, but that's probably all.
If you figure this out, you'd be the first.
Good luck!