Fixed nav menu on mobile

Site URL: https://gopher-mackerel-pen5.squarespace.com/

Hello SS brains trust,

Fixing up a few things on a clients site built by another designer and need some help with nav for mobile & tablet...

Nav menu is fixed and working perfectly on homepage, though when I navigate to the Team and Contact pages, it's not visible at all. Any ideas how to fix this?
It is already using custom CSS for the mobile nav, but I don't have the technical knowledge to figure it out myself.

site is: https://gopher-mackerel-pen5.squarespace.com/
pw: HIP2020!

Appreciate your help in advance. Muchas gracias.

RAD.