I think the basic problem here is that there's essentially no overlap between the LTE bands used by T-Mobile in the US and Softbank in Japan. Unless you have a

I think the basic problem here is that there's essentially no overlap between the LTE bands used by T-Mobile in the US and Softbank in Japan. Unless you have a device with radios that happen to support extra bands that are not used in the US, then it's essentially incompatible with the Japanese network despite both being "LTE".