A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
From Dahua Wiki
Network connection is not stable.
- There maybe an IP address conflict.
- There maybe a MAC address conflict.
- PC or device network card is not good.
- Check the cable connection from the system to the router/modem
- Your router or modem maybe failing and needs to be replaced.
- Check for DHCP and static IP address.
- Check MAC address.