I have the exact same problem.
The deal is, that T-Mobile basically is the worst cellular provider that I have ever seen- and I have had cellphones since 2001.
Used to sell them at RadioShack no less.
Way back then, the worst provider that we dealt with was Sprint.
Years later, 2018 to early 2021, Sprint was great, because they had changed their entire system.
Then T-Mobile bought Sprint, and now it has regressed to a point that is actually worse than what Sprint was back in 2001.
I actually feel like I have traveled back in time as far as cellular service.
The signal strength is laughable, and data is basically non-existant.
Even if I have 4 - 5 bars, the data is poop.
My only advice is to run- don't walk- to another provider.
AT&T is excellent, with Verizon in second place.
I base this on personal obsevations (I have had an AT&T phone since 2001, now as an emergency device because T-Mobile is so unreliable) and experience (when we travel my AT&T will often have service where her Verizon will not).
AT&T use GMS technology, which is basically what the rest of the world uses.
Verizon is switching, but slowly, from the antiquated system that they use called CDMA.
5G is a crapshoot, because if you are not close enough to spit on a tower you won't have any signal.