hi
The 'enable DHCP' server & 'obtain an IP automatically' are the correct options.
You shouldn't need to clone your mac for a cable modem, that is only required for using the TV set top box for internet access.
The next important thing is to switch everything off, then turn on the cable modem & let the lights settle until synch & rdy are steady, turn on the router & again give it a chance to initialise, then turn on the PC & you should pick up the connection.