Hey Tom,
The issue is because you have too many menu items in your menu. You need to reduce the number of menu items until your menu stays on 1 line before the desktop nav is hidden and the mobile nav button shows up.
Having lots of menu items is not recommended in web design and development. The theme was designed and developed with the best standards in mind so your current case is not supported by the theme. That means the theme does not have CSS that will cater for this case.
If you really need that setup, you will need to use Pro so can display a Navigation Layered element in desktop view in a Pro Header.
Thank you for understanding.