Welcome to the forums and for your first post - I was sorry to understand that you're having some issues bringing your number over to us.
In order to bring your number to us, your number must be and remain active with your previous supplier. You cannot have any disconnected requests on that number either or the port will fail.
If your number is no longer active with your old provider, we would advise that you need to contact them so they can activate it. (It sounds like you bought the number to Plusnet from BT so if you have now disconnected your account with Plusnet, your number would be returned to donor - which sounds like it was BT)
We are unable to request any re-connection with your old provider I'm afraid and we can only request numbers that are fully active.