If you were logged out of your account when you purchased a subscription, it got attached to your device.
In this case, you need to create an account manually:
Go to Settings.
Tap Sign up.
Go to Settings again.
Tap Restore purchases.
This way your subscription and tabs will be attached to your UG account and you won't lose them.
If you purchased a subscription, but the app doesn't recognize it, please make sure you log in to your Pro account.
Try all the emails that you might have used to create a UG account. The Forgot password option might be of help here.