Two-tiered navigation menu (for wp_nav_menu)

Hello,

Im trying to implement a two-tiered navigation menu in the Portfolium theme. This is a code I’ve used before to display parent pages and child pages in a two-tier way.

Two-tiered navigation menu

But now I would like to do the same but with the WordPress menu sustem.

So on row 2 I change “wp_list_pages” to “wp_nav_menu”. So far so good, it shows all my menu items.
But if I try to do the same thing on row 6 and 7 it does not show the children of the categories that I have in the “wp_nav_menu” list.

The pages I have in the “wp_nav_menu” still works fine with the child pages, but not my categories.

Maybe im missing something completely?

All help welcome!