Learn2 Menu Customisation

Still starting with Grav and no Twig experience, so apologies, this is probably a basic question.

I’m using Learn2 theme. I’m essentially trying to fake multi-site by having top level pages not on the menu. Access the top level page from the URL and then for the menu to show the sub-pages and sub-sub pages etc as the structure.

I tried, but just can’t restrict the menu to only display links belonging to the active top level page. If this is something simple enough and someone could post a snippet of some sort, I would be very grateful! Thanks!