Use Own Facebook App Problem

I’ve followed the documentation to add my own FB App and it seems out of date:

https://support.byonepress.com/how-to-register-a-facebook-app/

It does not mention the Valid OAuth Redirect URIs which seem to be required. I found a message on the CodeCanyon forum to use this url for the Redirect URI:

https://www.yoursite.com/admin-ajax.php?action=opanda_connect&opandaHandler=facebook

This did not work either, but if I edit the Redirect URI to the following it helps by resolving the errors:

https://www.yoursite.com/wp-admin/admin-ajax.php?action=opanda_connect&opandaHandler=facebook

Unfortunately, the Signin with Facebook button just redirects back to the locked content with “Error occurred during the request. Error validating client secret.”

I have double checked the client secret from the FB App and it’s correct.

I have also tried the Content Locker Pro plugin from MyThemeShop, and it works with my own Facebook App without requiring the Redirect URI. However, I cannot nest one locker inside of another with that plugin. Only this plugin has that functionality.

Any help here is very much appreciated.

The page I need help with: [log in to see the link]