Full-Width Layout – Header on top?

Hi,
we changed our page to the full-width layout, but we want the header to stay on top of the page when scrolling down.
this is our page: http://mypowers-esports.eu/
we want everything under the white navigation bar to scroll, and everything above (so navbar, logo, social etc) to stay at the top where it is..
i tried adding new divs to wrap around and make it position fixed, but it doesnt seem to work (using a child theme)