Usually, the cameras need to be in the same network as the VSS software (PC). However, it is possible to make it work when VSS and a camera are not in the same local network:
VSS uses ONVIF to find cameras, and uses RTSP to connect to a camera's video stream. If you don't know a camera's video stream URL, then you can temporarily put the camera in the same network as VSS. Assuming VSS can find the camera and its video stream URL, and that it works fine in the same local network, then you can move the camera to a different network. For it to work, you must make sure that VSS can connect to the camera's RTSP URL. Usually you need to edit the camera settings in VSS to update the IP address of the RTSP URL; and you probably need to configure your firewall/router settings to allow the RTSP connection.