I changed devices and want to purchase the subscription for it, but the system won’t let me do this. It only shows the old UDID on the main page. How do I register my new device?

If you have restored from a backup copy, try hard resetting the device. (Power+Home, or Power+Volume Down on iPhone 7 and later) After that, our system will obtain the new UDID. Login with your existing credentials and you'll receive an offer to register your device.