Enable Realurl to select individual errorPage

pageNotFoundHandling.php

<?php

class user_pageNotFound {
    function pageNotFound($params,$tsfeObj) {
        global $TYPO3_CONF_VARS;
 
        $url = \TYPO3\CMS\Core\Utility\GeneralUtility::getIndpEnv('TYPO3_SITE_URL').'index.php?id='.$TYPO3_CONF_VARS['EXTCONF']['realurl'][\TYPO3\CMS\Core\Utility\GeneralUtility::getIndpEnv('HTTP_HOST')]['errorPage'];
 
        $_buffer = $this->loadPage($url);
        echo $_buffer;
    }
 
    function loadPage($url) {
        $agent = "TYPO3 pageNotFoundFunction v1.0";
        $header[] = "Accept: text/vnd.wap.wml,*.*";
        $ch = curl_init($url);
 
        if ($ch) {
            curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
            curl_setopt($ch, CURLOPT_USERAGENT, $agent);
            curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
            curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
 
            $tmp = curl_exec($ch);
 
            if(FALSE == $tmp) {
                print_r(curl_error($ch));
            }
 
            curl_close($ch);
        }
        return $tmp;
    }
}

?>

Include 'pageNotFound_handling' => 'USER_FUNCTION:typo3conf/pageNotFoundHandling.php:user_pageNotFound->pageNotFound', in Localconfiguration.php FE section And setup in your realurl conf
$TYPO3_CONF_VARS['EXTCONF']['realurl']['domain.de']['errorPage']=PAGE_ID;