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".

The biggest issue is the phone bands, if your phone does not support the local bands, it will not connect.

I had no issue having a secondary phone in Japan w/ my T-mobile sim and having a local NTT sim. With dual sim/esim even easier nowadays to run both networks.

In international locations to your point it gets you enough for Maps and Mail, and some basic web browsing. You won't be streaming, that's for sure. However, if I need more than that, I use it to bootstrap an e-sim like GigSky which you can then route all data traffic to (at least on iOS) via Cellular settings.

This way you keep your phone number, iMessage, texting, and get 5G data too.

T-Mobile's free 128kbs is enough for whatapp, navigation and a few emails (Their MAX plan gives free 256kbs).

After basic troubleshooting T-Mobile requested that I call their international support on a 2nd phone and they would happily help me fix the problem with the original mobile phone.

Of course I didn't have access to a 2nd phone, I was livid. Cancelled the contract with T-Mobile and got local service. Even in cancelling T-Mobile couldn't get it right, they sent me two additional bills for trivial amounts in the months that followed. Each bill under a dollar.