Please ignore - i think I have solved the problem, and it was not actually to do with camera firmware. Instead, the update seems to have added a whole bunch more channels for my existing bullet cameras to be in rather than keeping them in their original allocations. Once I deleted enough excess channels to get back down to just 8, I was able to get the system functioning again.

I bought a new rlc-823A PTZ camera recently, to add to my existing system (an RLN8-410 NVR). The NVR would not recognize it initially. After I did a firmware update of the NVR, the PTZ camera could suddenly be seen - but then 5 of my previously working bullet cameras stopped showing up.

From what I can tell, I need to update the cameras' firmware too, via the (Update IPC option). Problem is, the download page indicates that I need to know the hardware version of my various bullet cameras, which I don't know. The reolink support pages show that teh hardware version can be obtained via the interface on the NVR, but since that stopped being accessible once the NVR was updated, just how on earth do I get this info ? I tried downloading firmware by guessing hardware versions for e.g. my RLC-410-5MP poe bullet cameras, but all attempts to upgrade them failed. Don't know if that's because I picked the wrong hardware version, or because the NVR isn't showing the cameras as being online to begin with. Seems like a hopeless chicken and egg situation. Do I have to try to install some older firmware back onto the NVR to get the cameras to be seen again, then try to update the cameras, then redo the NVR yet again ? What an mess..