All your purchases are bound to your Apple ID. To transfer your Premium between iOS devices:
- log into the App Store on your second device using the same Apple ID from which you made the Premium purchase. Then go to Menu > Settings > Restore Purchases.
- If you are on the start payment screen, simply tap Restore option.
And if you keep having these issues, please, send us an email to firstname.lastname@example.org, we'll help you restore the purchases.