Traveling to Japan

Hola RCQ925,

https://www.t-mobile.com/cell-phone-plans/international-roaming-plans
 

If you have T-Mobile you should be good to go international. See link above.

I used to be in the service and switched to T-Mobile because international travel was easier. Usually you just hop on their old network, e.g. instead of 5G you get 2 or 3G. So the speeds are very slow typically, but you shouldn't have to do anything like unlock your chip etc like AT&T and Verizon. 
 

Best luck and safe travels!