E. Marten

You need to use WordPress SEO by Yoast (Props to him!)
Adds “More Sitemaps” to your WordPress SEO Settings. If you are using a second WordPress-Installation in a subdirectory, you can now easily add the second Sitemap into the main one.
If you are using a second WordPress-installation in a subdirectory and want it to be fully integrated it into your main site to prevend struggeling with two or more Sitemaps in your Webmastertools, 
you can easily integrate more sitemaps into your main one. (sitemap_index.xml) 
The Plugin adds your second and third (aso) sitemap into the main sitemap and tooks care about the modification date automatically!
Thanks to Joost de Valk!

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension

Adds "More Sitemaps" to your Wordpress SEO Settings. Include differen sitemaps to your sitemap_index.xml

SeoMix

Daniel Roch

SEOKEY

WordPress 5.5 add core XML sitemap functionnality, but there is no user interface to disable it or to mange contents within theses files.
Disable WordPress XML sitemaps simply deactivate all WordPress XML sitemaps and redirect them to your home URL, preventing 404 page generation for Google and other Search Engines.
This SEO enhancement is already included in our <a href="https://wordpress.org/plugins/seo-key/" rel="ugc">SEOKEY plugin</a>. Not only do we deactivate WordPress native XML sitemaps (which are not optimized), but SEOKEY replaces them with much more powerful sitemaps (better crawl, not any harmful content).
More information here about it : <a href="https://www.seomix.fr/wordpress-xml-sitemaps/" rel="nofollow ugc">Core XML Sitemap Guide</a> (French)

SX Disable WordPress XML sitemaps

Disable and redirect core WordPress XML sitemaps

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension vs SX Disable WordPress XML sitemaps