Oh Jeez! My bad! We're talking about chapter 19 (which I still haven't made any progress in), not Sky Fortress. XD One day I'll be able to say I made some progress in chapter 19, but it is not this day! XD
The login screen has a tile the says 'Change servers' or something like that. You should have clicked it and from there it will show you your accounts. You would then tap on the account you wish to play on, and then you can log into that account.