If you already have an iOS iTunes subscription, you will need to restore your iTunes subscription before you can login (on any device).
- Open the new iOS app (you may need to update your app) and open the menu
- In the menu, select "subscribe"
- On the registration page, select "Restore from iTunes Purchases"
- You may be asked to sign into your iTunes account
- Once you have restored your subscription, you will be logged in and can enjoy all the benefits of your subscription