Why do ads appear after paying?

First, check if you are logged in with multiple Google accounts, as the app might have opened with one that doesn't have the payment assigned.

To fix this, uninstall the app, open Google Play, select the correct account from the left menu with which you made the purchase, then reinstall and open the app.

To ensure you are using the Premium version, open the side menu and check that "Remove Ads" option is gone and "premium" appears under the logo.