Hi again,
As you see this requires custom development and it’s not a default feature of the theme, this may cause issues that are unexpected and we won’t be able to support the issues caused by the above code we’ve provided.
With that being said I can point you in the right direction to fix your problem, since you’ve a fixed header in a desktop view opening a submenu will not show all the submenu items because of the fixed header behavior, you’ll need to change the fixed header to static header via Appearance > Customize > Navbar Position > Static Top this should solve the issue.
Again this is only the guide we’ve provided you as a custom code and we won’t be able to support the issues caused by this. I’d reccomend you to use Superfly plugin instead it does the same thing as well.
Hope this helps!