Dropdown gets hidden

Try this CSS:

.navbar-nav>li>.dropdown-menu {
  column-count: 2;
}

To add or override CSS: use the “Additional CSS” option in the customizer. https://codex.wordpress.org/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools to help you see and test changes to your CSS.

if that doesn’t work for you, either reduce the number of items on a submenu or use one of the megamenu plugins that lets you have multiple columns of a submenu

https://wordpress.org/plugins/search/mega+menu
https://wpmegamenu.com/