If you have purchased the Premium Service before, you can use it on your other devices for free, as long as they use the same Google Account.
Open the menu (≡) → "Premium Service" and tap "Restore" (for users who purchased before). The purchase is applied to that device.
If it doesn't work, make sure the account signed in to the Google Play Store app is the one you purchased with, restart the device, and try again.
If it still doesn't work, choose a plan and go through the purchase again. If you have purchased before with that Google Account, you won't be charged twice and the purchase will be applied.
In rare cases it still doesn't work. If so, create a backup on Google Drive from "Backup", uninstall and reinstall the app, restore your backup, and try again.