Thanks so far for the help.
I can now see both menus, but they don't really work together.
a)
When I set the side menu to only show menu items with level 2 and above, the entire side menu disappears.
Only when I include level 1 in the setting is the whole menu visible again.
b)
The side menu currently always shows all menu items. But that's not what I need.
However, only those menu items should be displayed in the side menu that are at level 2 or higher, depending on the selected main menu item (as can be seen in the pictures or on the website).
Kind regards
Wood