In this case, please check the connection between the router and the NVR. Unplug and plug the ethernet cable to make sure the connection is solid.
Then go to the NVR system, enable 'DHCP', "Obtain DNS Server Address" and 'Platform Access', disable 'DDNS' and check if the status of the NVR changes from "Offline" to "Online”. Please see the pictures attached as a reference.
If it doesn’t work, please try to reboot the system to see if the issue can be resolved.