I tried disconnecting my home internet, as suggested by VTTom, and here's what I found:
My initial concern was that Channels DVR app on my firestick would not be able to validate my subscription. but it turns out that was the least of my problems. The firestick will not do ANYTHING without an internet connection. Same for Apple TV.
My next thought was to use the Channels DVR built in web server. To keep my number of devices to a minimum, I decided to try running a WEB browser on the Raspberry Pi along with Channels DVR. I was running Channels DVR on Raspbian Lite, so I re-installed on the full version of Raspbian and used the Chrome browser on the Pi connected to the Channels DVR Web server, also on the Pi.
This worked, but it must have been too much of a load for my Raspberry Pi model 3; playback was choppy. So I went back to Raspbian Lite and plan to use a separate laptop or tablet to run the Web browser.