GSMT is T-Mobile NetworkHOORAY!! my TCL 10 PRO works with the GSMT sim from RedPocket but not with their GSMA sim i originally ordered. got it sorted on their chat, a little time consuming but being retired, no biggie IMO as i find that understanding people with accents an issue sometimes even thou not a fast typer its always understandable!!
Glad it worked out for you.