NVR Getting Disconnected Often

I had installed an NVR , SRN 1670D & connected 5 IP cameras to it . Sometimes , once in 10 days like that , the NVR gets disconnected from the internet. And when i power off the router & power it back after 2-3 min it runs fine .This problem is being faced by me often.

What could be the reason. Is it from the router or ??