Missing hamburger mobile menu

The hamburger menu icon is no longer visible on mobiles. The menu shows on desktops; just not on mobiles.

I have tried all the suggestions made in this forum: disabled plugins, made sure there is a header in place etc. I have 2 other sites, also using Minamaze (child theme) that have developed exactly the same problem so this strongly suggests that it’s theme related.

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