BT have now set things up where they want your BTID to be your BT email address and this is to be used to log onto MyBT and BTWifi and BT Sport.
To change it you need to log onto your MyBT and got to My Profile and change it from there.
When you log onto MyBT you will be on the My Bill page, if you look at the top under the BT logo you will see Buy our Products, "My Profile" is below that.
To change your BTID
Log onto MyBT
Go to My Profile
Go to My Contact Details > Edit
Change email address to your firstname.lastname@example.org
Enter present password for your current BTID
Go to My security details
Click on Password > Edit and change to the same as your email@example.com email password
Enter present password and click save,
Your BTID is now the same as your firstname.lastname@example.org email address and password.
This now how you will log onto MyBT, BTWifi, BT Sport and your email@example.com email.
Just to confirm: Your log on and password to MyBT is now your BTID and it is the same as your @btinternet.com email address and password.
Can you log onto MyBT with it here https://home.bt.com/login/loginform?TYPE=33554433&REALMOID=06-f1aca0f0-9277-1028-a2aa-843bdb4e0000&G...
Can you log onto your webmail with it here https://signin1.bt.com/login/emailloginform
If you can log onto both above can you check your status here and if need be opt in with the email address and password you used above.
I don't know why you can not connect so the moderators of the forum have been informed of your problem. Once they have read this they may be able to help. They are a BT UK based team and if they can help they will reply via this thread asking you to contact them via a link. Once you have replied to them by the link, it can at present take up to five working days for them to re-contact you.