Hello @fbaradmin,
Thanks for writing in!
I tried checking the website and can see that mobile menu is not working. I tried doing troubleshooting but it’s not working. However, I installed WP Server Info plugin and can see that you are using PHP version 5.6.38. I suggest you to update PHP to at least version 7 to avoid any compatibility issues. Please take a look at WordPress server requirement page. https://wordpress.org/about/requirements/
Although you have shared the login details, I am not allowed to do deep troubleshooting without taking due permission from customer. So I suggest you to please test for plugin conflict from Plugins > Installed Plugins and deactivate all the active plugins other then Cornerstone. After that clear browser cache and then try loading the website and see if mobile menu is working. If it’s working fine then you need to reactivate one plugin at a time to narrow down which one is causing the issue.
In case problem is still there, please give us the permission to test the website further. While we are testing, there may be some issues with the layout temporarily.
Thanks.