Can’t log in/import my account. Help!!

Hi all, I’ve recently decided to start up an alt account using a separate EA account. When I then try to import my main account back to my mobile (Apple) I get an error message that says it has to restart the game, and once it restarts, I’m still in my alt. I’ve tried unlinking Game Center, borrowing my partner’s iPad and attempting to log in from there, but I get the same error message

I’ve spent a lot of money on the main account and I’m very worried I can’t get access back to it. Any suggestions at all please? I’m getting desperate now as I’ve been locked out for several days now!