I am using my BT ID successfully on Windows 10 desktop to access the BT Account, My BT etc . . . and for years all OK...!
Now trying to login to iPhone apps for BT W-Fi and My BT and both fail.
Different reasons are given - My BT says request timed out, and BT Wi-Fi reports a credential error (and sometimes invalid certificate).
There does not appear to be any settings in the apps that can be configured before login so . . . it appears I am stuck in a loop!