How to make mobile submenu drop-down

Hi,
Having a hard time pinning down a title of my problem, so it’s also hard to search for a solution.
My problem is that my theme by default didn’t support a multi-level menu. I’ve added one, and it works now on computer, but once the screensize drops to the mobile-friendly menu size, the submenu items are displayed constantly next to each other instead of as a drop-down or submenu to their parent-items.
How do I go about fixing this? The theme is a child theme to a theme called Patti.
Here’s the page – the problem is the top of the two menus:
https://romowind.com/test/