It turns out that the previous owner of our house had several ADT NV214A video encoders installed to digitize the CCTV system. At first, I thought they’d end up being totally useless as the documentation online is pretty minimal and there was a password set from the previous ADT installation.
Luckily with a bit of digging, I was able to find this thread, which helpfully pointed out that you can reset the cameras by “Using a pin or paper clip, press and hold the reset button for at least 10 seconds when the power light is flashing”.
So after a couple tries, I was able to reset the device which then
opened up http://<ip address>/img/video.asf and yielded an
MPEG-4 stream. The manual also mentioned it had MJPEG support as well.
After a couple guesses I successfully found http://<ip
address>/img/video.mjpeg.
Hopefully this helps you on your quest to open up your ADT cameras.