After logging in on the t-mobile site and going to shop, selecting a phone and hitting the “upgrade” button, Brave does nothing. In Chrome, you get into an upgrade transaction ui.
Try it with Brave shields off. Does it work now? And please give an URL for reference.
It worked the next time I tried without disabling shield. Disabling shield on other problematic sites does usually resolve whatever problem I’m encountering. So far I’ve seen it on a university student portal site. And any url I provide for t-mobile.com will be specific to my credential, so not very useful to you.