Your subscription is applied to the account you are logged into in Simply Guitar when you subscribe.
Separate from your Apple ID, you need to be logged into this account in the app (including on additional devices) to access your subscription.
Tap Menu (☰) > Manage to see which account you used to log in
Make sure you are logged in with the same account you used to purchase your Membership (if not, please log in with the correct account)
Click on "Restore purchases"