Split: Sitemap not loading

I keep refreshing permalinks on 3 different websites but on neither of them i can get the Sitemap link to load anything.

Site n° 1 gives me this when i check Page Source with Firefox:

<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//apollo-cargo-alliance.com/main-sitemap.xsl"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
	<sitemap>
		<loc>https://apollo-cargo-alliance.com/page-sitemap.xml</loc>
		<lastmod>2020-03-05T08:17:25+00:00</lastmod>
	</sitemap>
</sitemapindex>
<!-- XML Sitemap generated by Rank Math SEO Plugin (c) Rank Math - rankmath.com -->

Site N°2 and N°3 give me something like this: 

<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//alfa-logistics-family.com/main-sitemap.xsl"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
	<sitemap>
		<loc>https://alfa-logistics-family.com/post-sitemap.xml</loc>
		<lastmod>2020-04-07T16:05:17+01:00</lastmod>
	</sitemap>
	<sitemap>
		<loc>https://alfa-logistics-family.com/page-sitemap.xml</loc>
		<lastmod>2020-03-05T09:47:59+00:00</lastmod>
	</sitemap>
	<sitemap>
		<loc>https://alfa-logistics-family.com/category-sitemap.xml</loc>
		<lastmod>2020-04-07T16:05:17+01:00</lastmod>
	</sitemap>
</sitemapindex>
<!-- XML Sitemap generated by Rank Math SEO Plugin (c) Rank Math - rankmath.com -->

So i must be doing something wrong on all 3 sites….

Can you please help me forward? Much appreciated.

Thierry