OrderByRelatedTo.php

<?php
// Element API endpoint
'news.json' => function() {
      return [
          'elementType' => Entry::class,
          'criteria' => [
              'section' => 'news'
              'join'      => [
                  [
                    'LEFT JOIN', 
                    '{{%relations}} as myrelations', 
                    '[[myrelations.sourceId]] = [[elements.id]] AND [[myrelations.fieldId]] = 9'
                  ],
                  [
                    'LEFT JOIN', 
                    '{{%content}} as mycontent', 
                    '[[mycontent.elementId]] = [[myrelations.targetId]]'
                  ]
              ],
              'orderBy'   => 'mycontent.title desc'
           ],
      ];
  },


// TWIG
{% set fieldId = 9 %}
{% set entries = craft
    .entries
    .section('awesomeNews')
    .leftJoin('{{%relations}} as myrelations', '[[myrelations.sourceId]] = [[elements.id]] and [[myrelations.fieldId]] = ' ~ fieldId)
    .leftJoin('{{%content}} as mycontent', '[[myrelations.targetId]] = [[mycontent.elementId]]')
    .groupBy('elements.id')
    .orderBy({'mycontent.title': SORT_ASC})
    .all()
%}