Sticky mobile menu

Hi!
I’m new and quite inexperienced with wordpress… anyhow, I built my recording studio website with shapely theme and am pretty satisfied…

BUT!…

I can’t find a way to make my mobile menu stick on top.

The desktop version works just fine, but it seems I can’t get it work on mobile.
Can you help me? Thanks!!!!

website is

https://www.blueberryrecordingstudios.com/

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