If you bought the Libra plugin prior to version 4, you shouldn’t be seeing ads in the latest version as an appreciation of your support over the years.
Note 1: purchases can only be honored on Android. At the moment there is no way to check for Google purchases on iOS.
Note 2: the plugin depends on Google Play to verify purchases which can be problematic in devices with modified system installations (custom ROM).
Make sure you’re using the same Google account on the device that you used to purchase the plugin.
Check if you purchased the old plugin which installs as a separate app. Try visiting the plugin page on the device where you installed Libra and install the plugin.
If you see an error message, that’s ok - it means you probably purchased the plugin from inside Libra.
Make sure you have a stable internet connection.
Open Libra, use the 3-dot button on the top-right to navigate to Settings. From there open the Subscription page and at the bottom tap the Restore purchases button.
At this point Libra will recheck if the plugin from step one is installed, and ask the Play Store for a refresh on the list of in-app purchases.
Look for the Google receipt from when you purchased your plugin, and forward it to email@example.com.