If you are experiencing difficulties when restoring your Premium, try to log out of your Apple ID then log back in.
You can do this by going to the device's Settings, then select your name banner, then scroll down through the options and tap Sign out.
After logging out, restart your device. Once the device has restarted, log back into your Apple ID and check your Premium.
If your iOS version is 10.2 or older, signing out works a bit differently: just go to Settings and select iCloud, then scroll down and tap Sign Out.