1ST > It’s Ok about clearing cache you can do that anytime If you use cloudflare for your CDN add a custom page rule “/sitemap_index.xml” and select baypass cache
2nd > Make sure you’ve defined already the pages, posts or CPTs to be indexed (avoiding increasing of 404).
3rd > Go to SEO > Sitemap > Uncheck “enable XML sitemap functionality.” > Save
4rd > Recheck again XML sitemap functionality > Save (regenerate xml file)
5th > Resubmit your sitemap_index.xml to webmaster > Refresh > Done
(note : if still not work you can try to submit your child sitemap.xml only one by one without the parent,ex “page-sitemap.xml”,”post-sitemap.xml” )