I hope that this post may be of help to someone. I sat down this evening to watch a show on my Channels DVR. However, none of my devices (multiple Apple TV 4k units, my MacBook Pro M1 or my iPhone) could find the DVR either over ethernet or wifi. Despite verifying the IP address, changing it from DHCP assigned to fixed IP and renewing leases, nothing changed. I spent over an hour trying to diagnose the problem. I went through my switches (one of which was just replaced under warranty, my router, changed a bunch of IP addresses again, shutdown my TailScale network, rebooted countless times and searched the internet high and low.
In the end, it turns out that the Windows Defender firewall rule allowing access to the Channels DVR on all networks (public, private, and domain) had been turned off. Neither I nor anyone else has logged into my HTPC that runs Channels DVR in many weeks. I have no idea how the rule was changed from 11 pm last night to 6 pm today. The only conclusion I can reach is that some update somewhere thought it was smarter than I am and decided to change the setting without telling me. I HATE developers and engineers who want to think they know better than you and change things like this behind the scenes.