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

Jeff Starr

Does one thing: disables the automatic WP Sitemaps.
Activate this plugin to disable the auto-generated WP Sitemaps (added in WordPress v5.5).
Features
<ul>
<li>Lightweight – only one line of code.</li>
<li>Simple to use – activate and done.</li>
<li>No settings to worry about.</li>
<li>Easy peasy.</li>
</ul>
Also stops WordPress from adding a sitemap rule to the virtual robots.txt file.
Why is this useful?
Because there are plugins that already implement sitemaps, and those plugins are used by a LOT of people. So all those sites with existing sitemaps will have TWO sets of sitemaps when upgraded to WP 5.5. Duplicate sets of sitemaps are discouraged as they may confuse the search engines. You don’t want to confuse the search engines, do you. So with WP 5.5 and beyond, you can either remove your existing sitemap solution or install this plugin to disable the WP-generated sitemap. Or maybe you have a site that does not need a sitemap. Or maybe you just don’t want a sitemap, that’s your business pal. Whatever the reason, this plugin will do the job with a click.
More infos
You can check your WP Sitemap located at <code>/wp-sitemap.xml</code>
Your sitemap should look similar to <a href="https://dev-tricks.com/wp-sitemap.xml" rel="nofollow ugc">this</a>.
For more information about the new WP Sitemaps functionality:
<ul>
<li><a href="https://make.wordpress.org/core/2020/07/22/new-xml-sitemaps-functionality-in-wordpress-5-5/" rel="nofollow ugc">New XML Sitemaps Functionality in WordPress 5.5</a></li>
<li><a href="https://perishablepress.com/wordpress-disable-wp-sitemaps/" rel="nofollow ugc">About the New WP Sitemaps and How to Disable</a></li>
</ul>
Privacy
This plugin does not collect or store any user data. It does not set any cookies, and it does not connect to any third-party locations. Thus, this plugin does not affect user privacy in any way.
Disable WP Sitemaps is developed and maintained by <a href="https://x.com/perishable" rel="nofollow ugc">Jeff Starr</a>, 15-year <a href="https://plugin-planet.com/" rel="nofollow ugc">WordPress developer</a> and <a href="https://books.perishablepress.com/" rel="nofollow ugc">book author</a>.
Support development
I develop and maintain this free plugin with love for the WordPress community. To show support, you can <a href="https://monzillamedia.com/donate.html" rel="nofollow ugc">make a donation</a> or purchase one of my books:
<ul>
<li><a href="https://wp-tao.com/" rel="nofollow ugc">The Tao of WordPress</a></li>
<li><a href="https://digwp.com/" rel="nofollow ugc">Digging into WordPress</a></li>
<li><a href="https://htaccessbook.com/" rel="nofollow ugc">.htaccess made easy</a></li>
<li><a href="https://wp-tao.com/wordpress-themes-book/" rel="nofollow ugc">WordPress Themes In Depth</a></li>
<li><a href="https://books.perishablepress.com/downloads/wizards-collection-sql-recipes-wordpress/" rel="nofollow ugc">Wizard’s SQL Recipes for WordPress</a></li>
</ul>
And/or purchase one of my premium WordPress plugins:
<ul>
<li><a href="https://plugin-planet.com/bbq-pro/" rel="nofollow ugc">BBQ Pro</a> – Blazing fast WordPress firewall</li>
<li><a href="https://plugin-planet.com/blackhole-pro/" rel="nofollow ugc">Blackhole Pro</a> – Automatically block bad bots</li>
<li><a href="https://plugin-planet.com/banhammer-pro/" rel="nofollow ugc">Banhammer Pro</a> – Monitor traffic and ban the bad guys</li>
<li><a href="https://plugin-planet.com/ga-google-analytics-pro/" rel="nofollow ugc">GA Google Analytics Pro</a> – Connect WordPress to Google Analytics</li>
<li><a href="https://plugin-planet.com/head-meta-pro/" rel="nofollow ugc">Head Meta Pro</a> – Ultimate Meta Tags for WordPress</li>
<li><a href="https://plugin-planet.com/simple-ajax-chat-pro/" rel="nofollow ugc">Simple Ajax Chat Pro</a> – Unlimited chat rooms</li>
<li><a href="https://plugin-planet.com/usp-pro/" rel="nofollow ugc">USP Pro</a> – Unlimited front-end forms</li>
</ul>
Links, tweets and likes also appreciated. Thank you! 🙂

Disable WP Sitemaps

Activate this plugin to disable the auto-generated WP Sitemaps (added in WP v5.5).

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension vs Disable WP Sitemaps