Hello @rfaass
yes, when I lookup the NB-IoT coverage of different providers the picture I get is that NB-IoT is available mostly in Europe whereas USA is mostly LTE-M.
IoT Creators seems to offer NB-IoT in the USA. It is my understanding they work with Deutsche Telekom which I guess is T-Mobile. So the T-Mobile SIM you ordered should hopefully work. If not, maybe check them out directly.
BTW: I think the SIM7020G is the most global variant SimCom offers; but yes, it's NB-IoT only. M5Stack offers a Unit and two Modules using the SIM7080G (NB-IoT and LTE-M) so maybe they are working on a variant that fits the M5Atom as well.
Thanks
Felix