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Are you using your BT ID or your email details to log in as this could be the problem? Check out this link What's my BT Wi-fi password or username? and let us know if this helps.
Have you tried logging in with your original btinternet.com address? As I understand it, other usernames don't work with BT WiFi even if they do with MyBT. It's never been joined up all that well.
For new users to BT to log onto BTWifi I believe that you need to use an @btinternet.com email address and that email address must be the same as your MyBT - BTID.
This is to make your BTID the same as your MYBT and BTWifi log on details so that you can access all the BT services through your MyBT.
To change your BTID you need to access your "MyProfile" in MYBT. Go to "My Contact Details" and change the email address to a @btinternet.com one and then go to "My Security" details and change the password to the same as you use to log into your email. This will make your BTID and password the same for all the BT services such as BTWifi and BT Sport.
You also need to make sure that your BTID is the primary BTID for that account.
I suspect the reason that your old phone works is because it has retained the old log on details in its cache.
You can check your BTWifi status here