Hey @Boans,
If you’re using Pro, edit your header and add two Bars (one for desktop screens and one for mobile devices) or to make things simple duplicate your Desktop Bar to have the same Bar setup and rename it to let’s say Mobile Bar (see screenshot)

Inspect your Mobile Bar and remove your current Navigation element and add Mega Menu element in replace of your Navigation element
In the Mobile Bar settings and go to Customize tab and hide it for large screens (see screenshot)

Now you need to hide the desktop bar in smaller screens, to do this repeat the above process for Desktop Bar and hide your desktop menu in small devices.

Alternatively the setup you need for mobile devices is already a part of original headers, so you can simply un-assign all of your headers in the header builder to activate the original headers that has the same accordion style mobile navigation (see live demo here https://demo.theme.co/integrity-business/)
Hope this helps!