Hi there, I have wifi router connected to a modem. When I start the router and connect cable to WAN port, it works well for couple of minutes, then WAN light goes red and after that disappears. By that time internet connection fails as well. If I restart the router, WAN light lights again and the cycle repeats. 

Any ideas what might be the problem? Modem and internet work well, even if router fails to work.

P.S. have reset, updated firmware - nothing helps