The sticky menu is intentionally disabled in smartphones lesser than seven inch due to space constraints. In iPhone, you can touch the top bar of the browser to instantly navigate to the top and then navigate to the desired location by invoking the right menu. This was the reason why both sticky menu and back to top button are disabled for phones.
In Android, I am not sure there is something like this available natively. If you need the sticky menu to be open for mobile browsers or want to at least have the back to top button, pls have a look at the js/main.js file at line number 473 (for back to top) and line number 82 for sticky menu (modify width conditional to something like 200 instead of 768). You will also need to remove the following CSS from css/responsive.css file at line number 93 –
#header.sticky { display: none !important; }