All your purchases are bound to your Apple ID/Google Account. To transfer your Premium between devices:
iOS: log into the App Store on your second device using the same Apple ID from which you made the Premium purchase. Tap Restore in Verv Profile settings or directly on the payment screen.
Android: make sure your Google Accounts match and you're using the same Google Account you used to purchase.
For now, there is no way to move purchases from Android to iOS (and vice versa).
And if you keep having these issues, please, send us an email at firstname.lastname@example.org, we'll help you restore the purchases.