Hey there MissMoody and Malderza,
Thanks for your time to reach out about this. That is odd that you both can’t get into a game on the mobile device. My first thought is something connection related and I see it mentioned also that the data connection was being used.
I’d like to see if you can both test using a wifi connection. Either your home wifi or a public connection should work to test. If my theory is correct, I think this is explained by the known issue on the forums post here. TLDR it has to do with Tmobile and how they manage IPv6 data, and the workaround mentioned in the forum post and talking to the mobile carrier would be advised while Blizzard works on a fix on our own end.
Hopefully this helps out. Cheers!