First of all, make sure that you're trying to restore access to the right app.
If you actually used to have Chords&Tabs app, then you can't restore the Tab Pro app, right? Tab Pro app has been removed from App Store, but you still can download it from your purchase history.
1. Log in to your Ultimate Guitar account. If you had access in the past and were logged in, then Pro access got attached to your Ultimate Guitar account. If you never logged into the app or don't remember the password or never knew that you could log in at all, please see the next option.
2. Restoring with your Apple ID.
1) Open the app
3) Open any tab
4) Hit "Restore" and enter the same Apple ID, that you used to make the purchase.