T-Mobile releases list containing names of all its 920+ markets that feature LTE-Advanced service
Currently, handsets offered by T-Mobile that are compatible with LTE-Advanced include the Samsung Galaxy S8, Samsung Galaxy S8+, Samsung Galaxy Note 8, Motorola Moto Z2 Force, and the LG V30. As newer flagship models are released, you can expect them to work with the LTE-A signals. Your best bet though, is to check with T-Mobile before making the purchase of a new phone.
To check out which of T-Mobile's markets offer LTE-A and LTE-A with the "Trifecta," click on the this link which will open a PDF file. The names of LTE-A markets are listed by states. Those with regular LTE-A coverage are in black text, while those with the enhanced capabilities featuring LTE-A with Carrier Aggregation, 4×4 MIMO, and 256QAM are printed in magenta (of course!).