This only applies if you signed up in the iOS app using your Apple/iTunes account.
If you've used the Restore Purchases option but it only seems to temporarily give you access to Player+, or if it changes back when you visit the 'Plan' section in the Settings Menu of the app, then this can be down to you not using either the original STV Account or Apple Account that you signed up with.
Your subscription requires both the Apple Account and the STV Player account details to match the original ones used to sign up , if only the Apple Account matches then although your subscription can be verified, it cannot be applied to the current STV account, so will only give you temporary access.
You can check which account is currently signed in by going to Settings > Account and checking the email address. You can also Sign Out from the Settings section and then sign in with the correct account and then you can Restore your Purchase and have it remain on your account.