If you have purchased the PRO version before, you may restore the purchase on other devices.
Note: If you don't see "Restore past transaction", it is okay to purchase again. You will NOT be deducted twice, even though it mention that you will be paying for the purchase. Apple recognise the purchase by the iTunes account and will not double charge.
- On the other device, tap on "Unlock PRO"
- In the unlock PRO screen, take on "Restore past transaction" at the bottom.
- Enter the iTunes account which you used to purchase the PRO version initially