Why doesnt the site work on any mobile devices
Omeka S themes are designed to be responsive. What is the url of your site and what mobile device & browser are you using?
The site you link to is not using an Omeka-created theme (one of these: https://omeka.org/s/themes/) You would need to work with your theme designer to adapt the theme they created to make it responsive.
If you switch that site to one of the Omeka Team themes, it should work on mobile.