What NVR model do you have and what firmware version is it running?
What was the old camera model and what camera model are you replacing it with?
Are the cameras you are replacing connected directly to the NVR PoE ports or to separate PoE Injectors/Switches on the local network?
Is the camera displaying the "The Network is Unreachable" error a different replacement model to the earlier ones that swapped without a problem?
I'm just trying to understand what you've done with this camera because it's not usually normal to manually add cameras to the NVR when you are plugging them into the NVR PoE ports.
You say you always use the same username & password for the cameras, so in the case of this new camera did you take it out of the box, plug it into a separate PoE Injector/Switch, set the username, password, and IP address using SADP, and then move the camera to the NVR PoE ports and manually add the camera to the NVR?
I think what has complicated things is that at some point in the past, someone has set some of your NVR channels/ports to 'Manual' and left some to 'Plug & Play'.
This was most likely because you or whoever was handling the NVR at the time was having some kind of issue with the NVR password that is used to activate the cameras and so most likely the installer manually configured some of your old cameras with a specific password in SADP and then added the cameras to the NVR manually using this password (which might not be the password you currently use).
Can you physically reach the new camera you are trying to add to the NVR or has the installer already put them up outside out of reach?
Do you have a separate PoE Injector/Switch that you could plug the new camera into?
Do you have a strong password set on the NVR? (don't say what the password is but just confirm if the password includes at least 3 of the following - lowercase, uppercase, numbers, and special characters)
I thought I was just about getting my head around how you had your system configured, but then I remembered something you said earlier on and it has confused me again: