The Device Network ID will be set by the DTH based on your configuration setting for IP and Port #. You can initially set the Device Network ID to whatever you want, but once you use the DTH for the first time it will be set to the correct value. You should not change it after it has been set on its own.
Have you configured the camera properly? You can go here to find more information on setup:
Yes, multiple cameras can be added. Each camera will have its own configuration settings. You can not reuse the same IP for multiple cameras, obviously. The exception to this rule would be using Dynamic DNS and port mapping, but that functionality is currently broken and I have not had time to work on it yet.