Sticky nav with Overflow Hidden template

Editing the SaaS Flow Plus template, and I’m trying to get this effect:

For some reason when I installed the template, the sticky nav was not available. Here’s our site:

Any solution for a sticky nav would be welcomed. The current nav div structure is Body > Overflow Hidden > Nav. If I remove Overflow Hidden (or :show)—it breaks several modules from the template that we’re currently using.

Here is my site Read-Only: LINK
(how to share your site Read-Only link)