mobile submenu sticky

Hey Magdalena,
Thank for your question, but the full-width submenu element is not designed to be sticky on mobile, in the element settings, the sticky option says that it is ignored on mobile.
To make the full-width submenu element sticky on mobile please add this custom ID sticky_on_mobile

Then add this css to your Quick CSS:

.sticky-top {
    position:fixed!important;
    top:0!important;
    z-index:1000!important;
}

Then add this code to the end of your child theme functions.php file in Appearance ▸ Editor:

function custom_sticky_submenu_on_mobile_script() { ?>
  <script>
window.addEventListener('DOMContentLoaded', function() {
(function($){
  $(function() { 
  	var width = $(window).width();
     var scroll_start = 0;
     var startchange = $('#sticky_on_mobile');
     var offset = startchange.offset();
      if (startchange.length){
      	if (width <= 989) {
     $(document).scroll(function() { 
        scroll_start = $(this).scrollTop();
        if(scroll_start > offset.top) {
          document.getElementById('sticky_on_mobile').classList.add('sticky-top');
         } else {
          document.getElementById('sticky_on_mobile').classList.remove('sticky-top');
         }
     });
      }
      }
  });
})(jQuery);
});
</script>
  <?php
}
add_action('wp_footer', 'custom_sticky_submenu_on_mobile_script');

Then clear your browser cache and check.

Best regards,
Mike