Unable to set sticky menu to desktop only

I want to have our main menu sticky / visible on scroll up but only on desktop views. I have set the sticky menu in the customizer to desktop only, but it is still showing on the mobile view. I don't have use nav as header or sticky header, so at a loss why it's still sticking on mobile.

Also, the menu has quite a lot of links and doesn't seem to want to scroll all the way down to the bottom on mobiles so the last items are not visible??

Cheers