If you use the same Apple Account, you can use the Premium Service on your other devices for free once you have purchased it.
Open the Settings screen, tap "Premium (No Ads)", and tap "Restore" at the bottom of the screen to apply your purchase.
If it does not work, check in the App Store app that you are signed in with the same Apple Account, restart the device, and try again.
If it still does not work, tap the purchase button once more to run the payment process again. If your Apple Account has purchased before, you will never be charged twice; a message will appear saying that you have already purchased it and it will be restored, and Premium will be applied.
In rare cases, none of this works, or the purchase button itself cannot be tapped. If so, please reinstall the app using the method below, which keeps your data safe while reinstalling.
Open the Settings app, tap "General", then "iPhone Storage", select this app, and tap "Offload App".
This removes the app itself while keeping its data on your device.
Then search for this app on the App Store, install it again, and check once more.
Thank you.