David,
I think the issue with your modem is the ability to open the UDP and TCP ports back towards the tenor, that is of course if you are setting up the modem as a router at the same time, but I really do not think that option is available on your modem..check it out, as for cisco, you should be able to use it as a router, I can not help you with the conf on the cisco but I am sure someone else reading this might do that, I still can not understand if you can get a cisco how come you can not get a dlink or linksys, they are both cheaper than cisco.
Hwyking