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…
Do phones still need tower info updates? Or maybe OP flashed a custom ROM so they’re not getting radio firmware updates OTA or something. There’s really not enough info here to go on.
My advice to OP: perform a factory reset if you’re on stock firmware, or flash the latest stock firmware if you aren’t. Cell phones are complex devices; there are quite a few components involved in cellular data.