add external Sitemap Link is not working

I am trying to add my own external links to RankMath buts looks not working, Tried with fresh installation and default themes

add_filter( 'rank_math/sitemap/index', function( $xml ) {
	$xml .= '
		<sitemap>
			<loc>http://example.com/new-sitemap.xml</loc>
			<lastmod>2020-09-14T20:34:15+00:00</lastmod>
		</sitemap>';
		return $xml;
}, 11 );