Hello,
Changing the default passwords of the cameras is definitely a good idea. When you plug the cameras into your NVR's POE ports (i.e built-in POE switch of the NVR), they are no longer accessible from the network - the web admin page of those cameras cannot be accessed by your laptop. So you have 2 options to access the individual camera settings pages:
1. Unplug the cameras from the NVR, plug them into your router or switch, use Hikvision's SADP software on a laptop to identify the cameras IP addresses. Then you can log into each camera using the camera's IP address shown by SADP and default password, and then change the camera's password. (I would do this).
2. If the NVR is not already on the same subnet as the rest of your network, change your laptop's network settings to match that of the NVR. Plug your laptop into one of the NVR's ports. Then run SADP and find all the cameras. Log into them and make whatever changes you want to make. (This is supposed to work but I cant confirm it does).
EDIT: See this post also
https://www.ipcamtalk.com/showthread.php/3292-Hikvision-NVR-POE-port-IP-address?p=27644&viewfull=1#post27644
Your cameras behind an NVR as only as safe as your NVR is. If a hacker can get to your NVR he can easily get to your cameras as well.
I recommend using a VPN to connect remotely to your home. This can be achieved in many routers or if you have a NAS (Network attached storage) system such as a Synology or QNAP.
Cheers