Unfortunately I’m not sure what’s going on. This is one of those customizations that would require extra time as there’s probably a lot more coding that would need to be done to get it working.
There could be a 3rd party plugin or custom code that is conflicting with it which is causing the issue. I’m not 100% sure. I’m seeing some unusual activity when I inspect the site which doesn’t show up on the demo. It could be coming from a script or something.
It looks like the mobile menu is opening up as it’s supposed to, but there’s not a clear way to close it properly since the toggle is hidden. This particular menu was originally designed to work inside the header.
Also, you may already know this but the Mobile menu isn’t really designed to be in a “fixed” position when scrolling down as some mobile browsers don’t support the “fixed” position. That’s why it only works on Desktop versions of most Genesis Themes.