do i add a switch?

You do understand that if you want to access your NVR from phone or somewhere else, then NVR needs to be connected to router.
You can do this indirectly, with NVR connected to switch, and then switch connected to router. But that means you may want a switch with more ports.

If you don't need access, then you can make a little stand alone network with the switch, cameras and NVR.
 
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