A firmware update was recently released for the NVR6XX-4KS2 series devices.
Information, instructions, and download links can be found Here
Troubleshoot/NVR/Troubleshoot IR Remote
From Dahua Wiki
Remote control does not work.
There are following possibilities:
1. If Remote control address is not correct then find the correct address.
2. If Distance is too far or control angle is too small then shorten the distance or fix control angle.
3. If Remote control battery power is low then replace battery power.
4. If Remote control is damaged or DVR front panel is damaged then replace remote control or DVR front panel.