Repair and modify wordpress menu

At the moment the page menu does not show subitems (in dropdown). It is also impossible to add new elements to menu in the backend. If you add new menu element, it just keeps loading and nothing happens.

First I would like to get these 2 issues repaired and as 3rd task, I would like to have menu like on this page: https://www.audi.ee/ee/web/et.html

First element will open megamenu and the rest are regular menu elements. Also - in mobile devices it should only show the first element of the menu (that opens megamenu).