Verify the App Store Apple ID matches the one used to buy premium.
Make sure the device has network access, enough storage, and sufficient battery.
Connect via a different network (for example, a hotspot), force-close Resprite, then reopen it and tap Restore Purchase. The app also checks for receipts automatically on launch.
Download any free app or sign out and back into the App Store. This refreshes the device receipt cache and usually allows the restore to complete.