Old 3com cable modem won't connet

It's a 3com OfficeConnect cable modem, it has both ethernet and USB connections, and the model number is 3CR29220 and/or CM29220 (both numbers are on the box), it is visually more-or-less identical to the HomeConnect 3CR29210; this modem is NOT of the "shark fin" variety that you'll see if you search for 3CR29220. It's brand new as far as I can tell even though it was discontinued probably 5 years ago (in other words, it has never been used). It powers up OK, the LEDs light up, but it will NOT connect. The ISP is Charter and I can find no info about this modem at either the Charter support site, or at the 3com site. I have the manual, and it basically says to run the ol' ipconfig /release and ipconfig /renew to get it working. Well, that doesn't work. Power cycling the PC and the modem doesn't work. Resetting Winsock doesn't work. Using the USB doesn't work. A friend gave me this modem 'cuz he never used it, and I figured I could save $5.99 a month by using this instead of the modem from my ISP that I'm "leasing". Any help will be appreciated. I don't have a phone (yeah - I know - I'm a loser) so I can't call my ISP for support. Even if I could call 'em, I know they'd say "we don't support 3rd party hardware not provided by Charter". Again - any help is appreciated.
THANKS!

(I'm currently using the Motorola modem my ISP gave me when I signed up; it works fine, but I'd like give it back to 'em if possible )

(oh yeah - I'm running Windows 7 Home Premium 64bit)

 

EDIT - I just looked at the box again, and there's a THIRD model number!! It's the same as the HomeConnect modem: 3CR29210 rev. AA