button on header page get hidden under mobile tray

It's only hidden because your mobile information bar is not disabled. When it's disabled you can see the button just fine. If you want your mobile information bar on, then I'd recommend shortening your mobile nav with this css:

.header-menu-nav-folder-content {
    flex-grow: 0; 
  text-align: left;
  
 
}

Let me know if this helps!

xo Danielle