The in-app purchase in Manga Rock for iOS is tied to your Apple ID. It means that you purchase and use the premium version without having a Manga Rock account. After you buy the premium version, you can restore it on any iOS devices for free. To do so, please follow these steps:
1. Make sure that your in-app purchase restriction is OFF
Step 1: On iOS device, open Settings > General > Restrictions
Step 2: Tap to Enable Restrictions. Enter the Restrictions passcode.
Step 3: Turn off In-App Purchases.
2. Restore in-app purchase
Step 1: Go to Device Settings > iTunes and App Stores
Step 2: Login with the Apple ID you used to purchase the premium version
Step 3: Open Manga Rock app in your new device
Step 4: Go to More tab > Upgrade to Premium > Restore
After that, enter the same Apple ID and password to restore the premium version.