Had similar problems with my P6P. There are multiple areas in my town where I show full / nearly full bars, but my service is terrible or non-existent (!). If I toggle Airplane Mode, I’ll get great speeds until it drops out again. Sometimes this is 30 seconds, sometimes 30 minutes, sometimes hours/days. But at some point it will drop out again. Ugh…
I don’t think the problem is TMO or any other service provider; it’s the way Android handles wifi and data. Whenever I have this problem, I turn off the Wi-Fi and suddenly everything works. Same when I am on wifi and shit isn’t working, I turn off the phone antenna to just have wifi and it starts working again.
There’s just an issue with it not switching over to the thing that actually has a connection and instead keeps trying to use the service that isn’t.
It could also just be that the tower you have full signal to is oversaturated by other customers and why you see slowdown or connection problems.
I think I had that problem going back to at least my HTC One. It’s still happening?