No sitemap gets generated for my WP multisite network

I have been trying the whole day to install ANY sitemap on my WP multisite network. I′ve tried almost all the multisite sitemap plugins I was able to find:

1. Simple Multisite Sitemap: After the plugin activation I′m supposed to see a sitemap unter https://www.myurl.sitemap.xml but it would only redirect me to my start page

2. RPS Sitemap: this is the only sitemap plugin that seems to work, by generating a sitemap05.xml into my root. But the plugin does not allow to automatically update the sitemap, which is what I desperatly need.

3. Better WordPress Google XML Sitemaps: once installed, it comes with a lot of setting options but also does not create a sitemap in the end.

4. WordPress SEO by Yoast: this would be my preferred plugin for my sitemaps! Once installed I can change the sitemap settings, but if I go to the plugin under SEO -> XML Sitemap and press the button for my “XML Sitemap” I once again only get redirected to my startpage.

So I search all over the web for a solution and tried both adding that code in .htacess and changing the init() code of the plugin file class-sitemaps.php as described in this tutorial here: https://www.wpbeginner.com/wp-tutorials/how-to-fix-yoasts-wordpress-seo-sitemap-404-error/ But none worked….the only change I had that when I changed the init() code in class-sitemaps.php I had one brace stet wrong the first time and the for the first time it would not redirect to my startpage but rather show a syntax error under myurl.com/sitemap-index.xml.

When I tested the WordPress SEO by Yost plugin on my single WordPress install, the myurl.com/sitemap-index.xml only returns a full page of black symbol gibberish.

I′d be happy if I could get that plugin to work on my multisite install. Please help!