whmcsDNSManager/modules/addons/DNSManager2/cron/cron.php

<?php

define('DS', DIRECTORY_SEPARATOR); 
define('WHMCS_MAIN_DIR', substr(dirname(__FILE__),0, strpos(dirname(__FILE__),'modules'.DS.'addons')));  
define('ADDON_DIR', substr(dirname(__FILE__), 0, strpos(dirname(__FILE__), DS.'cron')));

require_once WHMCS_MAIN_DIR.DS.'init.php';

require_once ADDON_DIR.DS.'loader.php';
$loader = new \MGModule\DNSManager2\loader();
    
\MGModule\DNSManager2\addon::cron();