Hi Anton L 

I'm also trying to create a page with a horizontal sub-menu and I have created a new menu as described, but don't see the "Site Navigation Submenu" style. 

As a result, I cannot add the menu to the header of my page.

The only workaround I have found is to add a 'menu' block to the page in the page builder. However, it's not ideal as it doesn't match the site menus

See screenshot attached. 

Any idea why this might be? or how to solve it? 

Many Thanks

Ian