Ensure you reset the network on the PC if your connecting it directly to the 6121. IT should get a IP address and connect to the modem. Modems web page is at 192.168.100.1.
Depending on some ISPs, mine, it will pop up in a browser that the modem has to be registered with the ISP. Mine asks for account info and then thats its. Turn off PC, modem, connect to router, reconnect PC to router then turn ON modem, then router and PC.