An Azure service that provides a platform-managed, scalable, and highly available application delivery controller as a service.
@Mohammad Moeini Thanks for reaching out. As your app service is connected as front-end pool to the application gateway every request that comes to application gateway will be redirected to your app service to serve the request.
Since you are blocking the frontend IP of the application gateway you are not able to reach the web site, and this is an expected behavior.
Hope this helps. If you need any further help on this do let me know.