First, please make sure you are signed into the same Google or Apple account on which you made your original purchase to remove adverts. Once you've confirmed you are signed into the correct account, open the game and tap the "Remove Ads" button at the lower right of the main menu. Tap the "Restore purchases" button, and the game should detect your existing purchase to remove adverts and apply it to your game.
If this doesn't work, please contact our support team for more help. As well as your message, please also include the email address of the account you made your original purchase with, as well as a screenshot of your receipt for your purchase to remove adverts, and someone from our team will be able to help you resolve your issue.