Ah, that’s why it’s not working with Firefox then too. Firefox comes with one of the secure DNS options turned on by default (DoH), which guarantees it will always reach a public DNS server and not get trapped into one from your home router, a cafe’s router, or your ISP. Since it knows the DNS will always be public, it also knows that the 192.168.10.20 address is not routable on the internet where it found it. S ome malicious sites can use a DNS record with a non-public IP address like this to get you to run JavaScript in your browser from the site you visited, to attack a device on your home network. So Firefox blocks that IP address from public DNS replies.

Generally people will have a home router that allows them to have their own recursive DNS where they can insert their own records to things within their home network, and will disable the DoH or DoT (“secure DNS”) settings in their browsers as the way to do this. Putting the private IP in the Public DNS record doesn’t hurt though, it just might get stopped by various modern security protections is all.