How to unlink city connected to Facebook?

I got the game on my iPad and linked it to facebook. It's an old ipad, and now I can't get into the game without it crashing on the loading screen. I've started a new (and better) city on my phone, but I'm unable to link it to Facebook. When I try, it says 'This account is already linked to another city. We cannot log you into this account.' I've tried going onto Facebook and removing the game from my profile, but that hasn't helped. Does anyone know how I can link my phone city to Facebook without going into it on my iPad to remove it?