problem with mobile menu

Hi everyone,

I’m experiencing an issue with the mobile version of my website, created with twentyseventeen theme.
In the mobile version the top menu is a collapsed button that became open when pressed. The issue is that when I scroll down, the menu has to change its position in fixed, so it can be sticky over the content while I’m navigating, but it doesn’t.
In the desktop version it works perfectly, and I know both the css and js to obtain this sticky menu, but don’t know where to put them in the code for the mobile version. I’m using a child version of the theme and am able to edit the template parts of the theme code.

Hope to resolve soon, thanks in advance for your attention!
Ab

The page I need help with: [log in to see the link]