For some operations, the CNS Server will send the device a URL to use in order to retrieve information such as configuration files, firmware upgrades, etc. The IP that is used by default is the Network Facing Interface, or the first ethernet interface listed when executing either ipconfig or ifconfig on the command line.
When NAT is being used, the address that the CNS Server will send will not work. In this case the Device Accessible IP Address in Admin/General should be set to an IP Address that the devices "see" on their side of the NAT.