I just signed up with T-Mobile and I don't know what's wrong with this ZenFone. It won't even connect calls in my house. My friend has the Droid Turbo 2 and it's the same problem. If I can't fix it I'll probably switch to ATT.
It sounds like your device may not be compatible. I recommend downloading this app: T-Mobile BYOD Check App to ensure compatibility with our network. Give that a shot and let me know what the app says for you.