network cable unplugged

I have just bought a Netgear WPN824 router. When connecting the hardware with the setup wizard the modem has to be turned off, which results in a message LAN network cable unplugged. When all cables are connected with modem-router-pc and it’s time to power up the modem again, the message LAN Network cable unplugged remains and there is no response from the modem. Therefore, the router is also not working and there is no internet connection.