Cable modem compatible with Back to my Mac?

Currently I have Comcast business internet with fixed IPs, but it turns out that the only that I use the fixed IPs is to log into my computers at home using afp:// and vnc:// while traveling. I would like to switch to Comcast for home in order to save $, but that will give me dynamic IPs.

Apple makes it seem that the modem/router needs to support UPnP or NAT-PMP in order for Back to my Mac to work, but I haven't found any cable modems that say that they support these protocols.

Any suggestions for using Back to my Mac with a cable modem and dynamic IPs before I change my service?

If it makes any difference, currently my home computers are hooked up to the outside world by ethernet, using an Airport Extreme as a switch between the modem and the computer.