Test with a computer connected to the router by ethernet. If that also disconnects then test with a computer connected directly to the modem. (Remember to power cycle the modem when switching the device connected to it.)
That modem has pretty good LEDs to let you know what's happening, and the router may also. Haven't you tried "reading" them?
That modem has pretty good LEDs to let you know what's happening, and the router may also. Haven't you tried "reading" them?