DS-7608NI-E2 cannot be connected externally by ddns in ivms?

Hello everybody,

I have a Hikvision DS-7608NI-E2 DVR.

now I have set up DDNS and used dyn.com.
Connection okay. I also activated port forwarding and changed the HTTP port to 7899.

Now the receiver can be reached at my brother via dyn.com:7899.

However, the DVR does not allow you to be integrated into ivms4000 via IP.

The error "Device offline" always occurs

However, the port obviously leads out, otherwise my brother could not access it.