As long as you use the same Apple Account, anyone who has purchased the Premium Service can use it on other devices at no extra cost.
Tap "Premium Service" in the settings screen, then tap "Restore".
If it doesn't work, check that the Apple Account set in the App Store app is the same one, then restart your device and try again.
If it still doesn't work, tap the "Purchase" button once more and go through the purchase process again. If you have purchased before with that Apple Account, you will not be charged twice. You'll see a message confirming that you've already purchased the item, and your Premium Service access will be restored.
In rare cases the app still doesn't work properly, or the Purchase button can't be tapped. If so, follow the steps below to offload and reinstall the app while keeping your data intact.
To do this, open the Settings app, tap "General" → "iPhone Storage", select this app, and tap "Offload App".
This removes the app while keeping its data on your device.
Then find the app on the App Store, reinstall it, and check whether you can restore your purchase.
Thank you.