I recently bought a Dahua NVR(DHI-NVR5216-16P-4KS2E) to replace old Dahua 4216. About half of my cameras will not connect to the 5216 although they work perfectly with both the 4216 and BlueIris. I have upgraded the 5216 to the latest firmware but that did not help. The cameras that do not connect are all Dahua purchased around 2015. Out of 10 cameras - 6 do not connect. I checked and double checked all the userids and passwords. The cameras are all connected to the LAN via POE switches. I am not using the POE connectors on the back of the 5216. Any thoughts on what might be wrong or how to troubleshoot this?