I am in the military and just got deployed to Okinawa Japan. I had Verizon s7 and asked about a service plan and my local cell store sold me on a T-Mobile plan, where all I had to do was buy a T-Mobile SIM card and once overseas, just install it. I suspended my Verizon plan for six months like I was told. Soon as I arrived to Japan, installed the SIM card, and its a no go. Please help, thanks.
Unfortunately, "it's a no go." does not give us a lot to hang our hats on. I think you may have made a mistake by not testing your T-Mobile account in the USA before going to Japan. I think usage in the US is required before going abroad. You may have to send your SIM back to the US and have someone use it there and send it back to you. If the phone says "Invalid SIM" or something like that, it may be locked. I understand that Verizon has figured out a way to get around the no-locking requirement of band-13.
ITMT, somewhere on base (exchange, USO, food court, etc) there is an agent who will sell you service on a Japanese carrier. Don't ask in town because the stores there don't know how to sell to non-residents,
Manage your T-Mobile account