I’m sorry, but I think there is a misunderstanding here. The theme’s mobile menu should work perfectly fine. You just need to click the “Menu” button at the bottom of the screen.
The theme displays 2 menus in header: primary and secondary. The primary menu is the main navigational menu of your website and it has a mobile version too (the “Menu” button on mobiles). The secondary menu does not even support submenus in desktop view of the website. You can read more in theme documentation.