A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
From Dahua Wiki
There is no HDMI output.
- If Display is not set in HDMI mode then set to HDMI.
- If The HDMI connection is not fully set in the connection then configure and reconnect HDMI.
- If The HDMI connector is damaged then change the HDMI Cable.
- If HDMI Cable is damaged then change HDMI Cable.
- If Check if monitor is on the correct source setting then set to the correct source.
- If DVR / NVR is set to a resolution that is unsupported by monitor then reset resolution.