After years of never having an issue, I just recently have not been able to connect Off-LAN. Any ideas?
are you using this link?
my.channelsdvr.net
I always typed in the server IP address manually but using your link gives me the exact same landing page:
strange. i guess when you're at home wifi, try it from there next so we can strike out any issues with the server itself.
Works fine on the Lan.
the oneUI update the other day seemed to mess up tailscale for me. I had the correct ip, but for some reason once I copied to clipboard from the tailscale app and pasted into channels it worked. 
i was using the android app though.
humor me -- when you can access it from LAN, go to CDVR admin page, then General, then Remote Streaming. Turn it off, and on, then test the connection.
Toggled it a few times didn't make a difference.
Downloaded Brave browser on my phone, entered the IP, it took me to the authorization page, got it and boom I was in.
So something to do with Chrome browser on Android I guess.