Do you know for certain the NVR is working?
Is it possible you are port forwarding and your IP address changed?
THANKS FOR RESPOND, THE NVR IS WORKING AND ITS CONNECT TO ROUTER ( INTERNET ) ON MY OFFICE, NOTHING CHANGE AND EVERYTHING WAS OK , I RESET MY ROUTER AT HOME AND SINCE THEN THE SMART PSS ON MY DESKTOP SHOWS OFFLINE
ALL MY CAMERA IS OEM DAHUA 8MG 4K AND Nvr IS 8CHANNEL 4GB 4K. I CAN NOT SEE MY CAMERA ON MOBILE APP TOO.