Yes. The “USA” in “Librem 5 USA” only refers to where it’s manufactured, not where it can be used.
In fact, T-mobile (along with MVNOs that use T-mobile’s infrastructure) is probably the only U.S. carrier where you can currently get service for the L5.
Lots of us U.S. forum members here are using T-mobile or an MVNO on T-mobile.
Sometimes it helps to go into Settings > Mobile, then click on Network (which should be showing “T-mobile” - if it’s not, then that’s your problem - and also make sure “2G, 3G, 4G (Preferred)” is enabled), then click “Set,” even if it’s already showing “T-mobile” as selected.
Under “Access Point Names,” make sure T-mobile is selected, click on the gear wheel, and make sure the APN is set correctly, then save it.
You will also need to have this relatively new app installed, with VoLTE enabled: List of Apps that fit and function well [Post them here.] - #313 by amarok