Mobile menu not working

Since upgrading to “Twenty Seventeen ver. 3.5” the mobile menu on the site has stopped working, although the normal menu works fine. On an iPhone in portrait mode the menu button does nothing but rotate it to landscape view and the normal menu appears and functions fine, likewise, on a pc the menu works but if you drag the browser edge to make it a narrow window until the mobile menu appears it stops working.

I initially found, by disabling the plugins, that it seemed to be the “Really Simple SSL” plugin but I also found that if I re-enabled the plugin and switched to another theme, either “Twenty Twenty-three” or “Twenty Twenty-four”, then the mobile menus on those themes worked fine which makes me think that the problem is within the theme and not the plugin. All plugins and themes are using the latest versions. Any help would be appreciated, thanks.

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