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…
Google has been using a bad modem ever since the Pixel 6, resulting in poor cellular connectivity. Although given how strong the connection looks, this is likely at least partially due to congestion on T-Mobile’s network as well.