Transferring account

Hi

I have a second account on my old iPad, that is now not supported by Swgoh
I have just downloaded the game on my computer at home to play on bluestacks eg
How can I sign into it across different platforms? Obviously I cannot sign in on the iPad anymore as it won’t let me

I already have my main account on my phone that is signed into gamecentre, my alt is not

Any help Is appreciated