Everything works, but the problem is that on my main LAN network the server is running on a public IP on the public domain www.example.cz. Which is accessible for example via my mobile data. But not through my newly created Guest network why?
We'd need to see your configuration -- it's likely a firewall issue, but we cannot know without more info.
Please copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have: