To use BTWifi you have to log on using your BTID.
You may find that your BTID needs to be an email address for it to work with BTWifi.
You can check your BTWifi status here
this will show if your BTID is been recognised by BTWifi.
Unless there is a different method for BT Mobile users, to use BTWifi via the app you have to log on in the method I posted above. The link I posted will tell you if your log on details are being recognised by BT.