I connected the laptop to the NVR and was able to see 7 out of 16 cameras.
To the LAN that the NVR LAN port is connected to?
Or to one of the NVR PoE ports?
With SADP or with Live View on the NVR web GUI?
Presumably on the LAN with the NVR web GUI, or you would see all 16 (15 if the PC is using one port) with SADP on an NVR PoE port.
Connect a laptop to an unused NVR PoE port.
SADP will find all the cameras.
Change the PC IP address to be in the same range as the cameras, usually 192.168.254.x
Double click the IP address for each camera in the SADP status screen.
Log in to the camera using the old NVR password to access the cameras.
Did you try this?
Double-clicking the IP address shown by SADP launches the default browser, which will bring up the camera login page.
At this you can test the admin password, confirm if it's using the old NVR password.
Also - check what firmware version SADP shows for the cameras.
If it's 5.4.0 or earlier, the camera password can be extracted using a firmware security vulnerability.