Ultimate Guitar Pro is a cross-platform service.
To use your Apple subscription on multiple platforms and devices you need to log in to the account your subscription is attached to.
Note: If you registered with Google, Facebook, or Apple, you need to use one of these accounts to log in.
Can't log in
If you were logged out of your account when you purchased a subscription in the iOS or Android app, it got attached to your device. In this case, you can access Pro only on this device.
Cross-platform subscription
To make your subscription cross-platform:
Open the mobile app.
Tap on the settings icon.
Sign up.
Go to Settings again.
Tap on Restore purchases.