Order Ajax Load More results by custom meta_query clause.

functions.php

<?php 
// @see https://connekthq.com/plugins/ajax-load-more/docs/filter-hooks/#alm_query_args
function my_filter_args( $args, $id ){ 
   // [ajax_load_more id="my_alm_id"]
   
   // Custom meta query clause.
   $datum_clause = [ 
      'key'     => 'datum',
      'value'   => date("Ymd"),
      'compare' => '>=' ,
      'type'    => 'DATETIME' 
   ];

   // Add to meta_query.
   $args['meta_query']['datum_clause'] = $datum_clause; 

   // Add custom ordering.
   $args['orderby'] = [
      'datum_clause' => 'ASC',
   ];
   return $args;
}
add_filter( 'alm_query_args_my_alm_id', 'my_filter_args', 10, 2);