The reason you are having difficulty is because we use the Mobile App Industry’s Best Practice to process our subscriptions and payments. This means that we sync your Apple ID or Google Play Store ID to the device used to subscribe. This is designed to prevent multiple people sharing a single subscription.
For the app to work as intended on your iPad you will need to purchase a second subscription. This is because Apple and Google are 2 separate companies that support two separate devices. Its a bit like having DVDs and BluRays.
Its the same movie but you cant play a BluRay in a DVD player and vice versa. The Android app is owned by a different publisher, Google, and the Apple App is owned by Apple App store.
You could choose to cancel the app on your phone with Google and use your iPad as your app device Or, again vice versa.