I've been trying to view my bill via the app but it just says "sorry, there was a problem" same as the your usage tab? also tried logging into my account via pc but just says that my details couldnt be found ?
I would perhaps try a password reset on the PC and make sure your browser is up to date. If you still cannot log in then customer care may need to do something with your BTID to get it fixed
try the message us and see if that helps you
https://www.bt.com/help/contact-bt/account-and-billing/broadband