My condo building's Lobby Watch camera is available on channel 955. I'm the techie for my upstairs senior neighbour. She has a 9865 PVR in the livingroom and, in her reading nook, a new 4K Nextbox PVR which replaced a previous dying PVR in October.
She can see the Lobby Watch camera on the 9865 but, on the new Nextbox, the image is black. There is no message similar to "unauthorized channel". Why would she not get a picture at 955 on the next PVR?
Try swapping the boxes to see if the boxes work in the other location. There's no reason the 4K box shouldn't work. If the 4K box works when swapped, then there's some sort of signal issue to the location that doesn't work. If the 4K box doesn't work in the known working location, try a reboot of the 4K box (which will likely happen when you move the box anyway...).
Different boxes can require different signal levels. Try a reboot. If that doesn't work, try the swap. You can also try checking the signal at that box to see if it's significantly different than the other box. Are other channels "black".
You can also call Rogers to have them check the signal.