So, I just ran into this issue recently and was able to slog through a solution that I thought I would share. In brief I was doing a twenty camera install (G3 Bullets) with a UNVR (which are just a fantastic bargain, seriously). 3-5 of the cameras would just go into some kind of zombie mode after initially starting the adoption process and running the firmware update. The NVR just couldn’t see them. So, here’s the two things you should do if you run into this: 1) make sure the NVR is plugged into the same physical switch as the cameras. 2) if the NVR can’t see them, log into them individually by their IPs in a browser tab, and manually enter the NVR’s IP. Return to the NVR’s UI, and do the “Add item” button, and the camera will magically show up to be adopted.
I couldn’t find this documented anywhere, but came across them as unrelated suggestions while researching, and this combination helped me out of the problem.