The fixed to the bottom icon is in fact what I’m referring to. When a site is accessed via a mobile device(or tablet) the icon does not show. I’ve also tried to use the setup option that is supposed to keep the icon visible if another element overlays it, and had no success.
A quick test you could try is to reduce your desktop browser to a tablet or mobile screen size. I’ve also accessed the site with a Pixel cell phone with no success in getting the message icon to show.