On click this will check the window size and if it is smaller than 500px it will go to the modal page otherwise will show the popup

only-fire-function-window-size-is-less-than

jQuery(".open-modal").click(function(){ 
          if (jQuery(window).width() < 500) {           
            window.location = 'http://www.themodalpage.com';           
            }
        else {                      
            jQuery("#modal").fadeIn();
            jQuery(".modal-overlay").fadeIn();
            }
         });