Your ./wp-sitemap.xml should show some URLs and text about the XML Sitemap is generated by WordPress. But It doesn’t and how can you fix the setup?
Do you use a Child-Theme and your XML Sitemap isn’t reachable?
The URL for the new Sitemap is [your-site’s-url]/wp-sitemap.xml. After empty the browser cache, load the sitemap with other browsers or wait hours and days. Nothing is happened about loading the WordPress XML sitemap ./wp-sitemap.xml. What’s going on?
Have you a Child-Theme in use? If yes, activate the Theme your Child-Theme is inherited. Type your Sitemap URL to your browser address bar [your-site’s-url]/wp-sitemap.xml and wait at most 5 minutes and the WordPress 5.5 XML Sitemap is shown. After the Sitemap is shown switch back to your Child-Theme and activates it. If you don’t want to wait for a short Theme change between both Themes can have the same effect.
Switch the Theme in the Admin Setup Bar
To change the Theme go to the Dashboard. In the Admin Setup Bar > Appearance > Themes you find the active Theme and all other installed Themes. With the mouse over the Theme’s preview image there appears a button “Activate” you should click.
Sub-Domains can also use the new wp-sitemap.xml
Also, your subdomains should work well, after activate once the Theme where your Child Theme is inherited and change back to your Child Theme.