I'm on T-Mobile, Lumia 635
I don't think I have 8.1 GDR 2... I have 8.10.12400.899 installed
When I open the app it says No Builds... I then go to Make sure you're registered... I choose alternate login (even though it's my phone's primary already) and I choose Microsoft Account, then I login... it then says "It doesn't look like you're registered with the Insider Program..." and gives me a link to re-login and join... but I'm obviously already registered or I wouldn't be posting here.