A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
From Dahua Wiki
My IPC is off line from time to time.
- Check if the network environment is good and the network cable hole plug of IPC end is loose.
- Check if there is IP conflict.
- Check if the device is under normal power supply.
- Check if the device upgraded to a wrong program.