What error does the NVR show next to the cameras? You can see it in the camera list in the web interface or on the local monitor, and the message there usually points pretty directly at the cause.
Also, are the cameras plugged into the NVR's PoE ports or into a separate switch?
If they're on the PoE ports, SADP only showing the NVR is normal, since the cameras sit on the NVR's internal network and SADP on your LAN can't see them. In that case the ISP change is worth a look. The NVR's internal PoE network defaults to 192.168.254.x, and if the new router hands out addresses in that same range, the NVR can lose its cameras. Compare the new router's LAN range with the Internal NIC IPv4 address in the NVR's network settings, and if they overlap, change one of them.
If they're on a separate switch, SADP should normally still find them even if they're on a different subnet, so not seeing them at all points more to power (the switch or PoE injector) or cabling than to the NVR.
