I still don’t understand what you mean. The navigation of the site you linked is pretty regular, so I’m not sure what difficulty you are running into.
I can’t make the website show more than one separate sub menu
Not sure if I understand correctly, each sub-menu has a parent menu item, so you can build something like this:
Parent menu 1
- child menu 1
- child menu 2
- child menu 3
Prent menu 2
- child menu 1
- child menu 2
- child menu 3
- child menu 4
Prent menu 3
- child menu 1
- child menu 2
- child menu 3
- child menu 4