TLDR: I can stream remotely via web, but iOS app fails to connect remotely.
SOLVED see below.
Port forwarded manually (8089, verified with port checker), able to load my.channelsdvr.net and stream via the web (over LTE on my iOS device as a test). When opening the app and selecting "connect to channels DVR" then "Away from Home" (again on the same web-playing LTE device), I get the "Your Channels DVR Server at home can not be reached."
Tried logging out and logging back in, and reinstalled the app (actually, I had been using the original Channels app, so I deleted and installed the Channels DVR app instead).
Log shows this:
2020/01/22 08:46:02 http: TLS handshake error from 52.207.12.176:48654: remote error: tls: expired certificate
Solution
EDIT: Updated to 2020.01.19.2104 version (noted in another thread to click-and-hold the update button in the web interface) and it STILL didn't work. Force-quit and reopened Channels DVR app on iOS and it started working