You may be able to access the RTSP streams from the camera, perhaps in VLC, but I would do as I suggested so you can use the full functionality of the PVR
If the cameras a need POE, the RT2600 will NOT provide it, so you will need a POE injector or a POE Switch, probably about the same cost. Personally I would use a POE switch as it's a bit tidier.
If you're going to create a mesh, I would have thought moving the PVR inside would be the best plan. i.e.
Garage has RT2600, POE switch and two cameras.
Basement has NVR, RT6600, Mac
If you're thinking of ditching the NVR, I have used a Synology DS120J NAS box to run a couple of cameras on Surveillance station without issue, you can often. pick up DS120J on eBay for <$90.
I currently use a DVA1622 with 12 cameras, and I use the Mac client on M2 Macs.