I was in my phone
I went to the United app, clicked "wifi & entertainment" and there was a t-mobile button.
That option wasn't availability in my laptop, but I just used the same URL and got it to work (using a different number)

Everytime it's been extremely slow so not that useful. It says it supports video streaming, not sure how that's possible when it can barely load the login page

I guess some Internet is better than zero