Went from 3 bars to 1 after upgrade

Yes there can be a multitude of variables. In my case after multiple support calls to T-Mobile including a tower technical case and Apple support for our phones (13 pro and 13 mini, no issues and current), the bottom line is in our area T-mobile through multiple tower maintenance upgrades has deemphasized 4G LTE in favor of 5G. The result is in our home, 4.5 miles between two T-Mobile towers, we went from 2 bars of reliable calling and texting to 0 bars, SOS, or an unreliable 1 bar if we're lucky. At our distance 5G  is not available within our home.  T-Mobile's solution at our home   was to offer a hot-spot that, in their words, requires a land internet connection which we don't have or want. They can't use our T-Mobile Home Internet - which btw is not a good as pre tower upgrades. 

Since we also have seen less reliable coverage at other geographic locations in our rural area, interference isn’t the cause at our home.

As a loyal long time T-mobile customer we're left with the decision to stay with the current less than marginal service at home relying entirely on wifi calling, try third party signal boosters, or change to another carrier. Yes I've tested another carrier via dual e-sims on both our phones and we get reliable cell service with the other carrier within our home.

Trust me I know a good deal about troubleshooting complex problems after a 3 decade career in IT and have tried everything suggested by T-Mobile and Apple Support and other sources.

If you're close to a tower, T-Mobile is fantastic. In the marginal areas not so much.