I just tested on desktop (by shrinking screen down) and on an iPhone and I can click both the + and user icon and both work for me. I can see the login module slide down and the register one too. Maybe try clearing the cache on the mobile you are using? Not sure what else would cause it not to show for you.
Thank you but I fixed the issue by disabling the custom login module with the "click here to register" link. With that published the login forum wasn't appearing.