Can't connect DD-WRT Router to Comcast

[Jim]

  FC13 / KDE-4

I cannot get a Linksys WRT54G with DD-WRT  or a Linksys WRT160N to 
connect to a Arris TM602G/CT Modem.

1. I have called Comcast and they swear that the connection is not 
registered to my PC MAC address.

2. I cloned the PC mac address in my DD-WRT/Router.

3. I have pulled the battery out of the Cable Modem , reset, 
disconnected power cord and ethernet cable and wait 1 minute before 
plugging modem back in.
     And put in ethernet cable and powered modem back up.

4. I then powered up the Router and reset it and I get no WAN IP 
addresses in router.
     PC connects to router and I can login to router and check settings, 
No WAN Ip's in router.
     I also Cloned the PC Mac address in Router .

5. I know for sure that the WRT54G is working because I have already had 
it connect to this Modem, but I had to move everything to a different room.

6. I know the Ethernet Cable is okay because, I can remove it from 
router and plug it directly into PC, and reboot PC and I have a Internet 
Connection, without having to pull the power plug on Modem
     The ethernet cable is 25 ft. long because the Modem is up stairs 
and I can't relocate it , but that should not be a problem.