Using IP Cameras from store bought system with homemade NVR

I bought a system from NightOwl ( http://nightowlsp.com/products/complete-security-systems/hd-security-systems/4-channel/nvr10-441.html) which is working fine with its current setup. However, after installing everything, there are a few spots around the house I wasn't able to capture. Instead of returning the NightOwl 4 port and getting another with 8 or 16 ports, I am trying to use the IP cameras that came with it on a home built NVR I am setting up with BlueIris..

I can see the IP addresses and ports listed in the nightowl NVR application but any way that I try to connect to them directly is failing.

RTSP works fine when using the IP address of the NightOwl NVR but since I will be taking that out of the equation, I need to find out how I can access these IP cameras directly through other software.

Any thoughts on how I should approach this?