Multisite + Subdomains not working . Just 1 Subdomain correctly pointing

I setup a multisite on wordpress as subdomain .

Now when I point dev.mydomain.com to the server (default) it works.
If I create a second A record with demo.mydomain.com and point it to a site and rename the URL to that name it shows a 503 error and is not reachable.

If I point a demo.anotherdomain.com A record to that site and change the URL it works.

How can I make it work that multiple subdomains of a primary domain work? I want to create different demo wordpress sites.

Another thing is how to setup letsencrypt for the multisite network and if there’s a best practice?

Thank you so much for helping!