thanks for your reply.
as you suggested, I disabled the caching system and then I tried to add the filter, but without luck.
add_filter( 'the_seo_framework_sitemap_additional_urls', function( $custom_urls = []) {
$custom_urls[ $url ] = [
'https://example.com/somepage' => [ 'lastmod' => '2021-12-16 10:58' ],
];
return $custom_urls;
} );
What I am missing or doing wrong?
moreover, do you know if it is possible with this filter instead of including just one page, to include directly also a second external sitemap (and keeping the default sitemap that your plugin creates)?
thanks a lot for the plugin