You can use your own modem with Comcast business service (I do) but it took about two weeks of calling all the tech support numbers I could find until I finally got a tech who was able to do it. Not because it's hard to do, but because most of the people I talked to were lying or incompetent.

The guy seemed a bit weirded out at how profusely I thanked him -- after all, all I'd done was tell him the model and MAC address of my generic Motorola modem.

Don't waste your time explaining to them why you want to use a real modem (I have my own router/firewall, their modem had NAT I could not disable) or that it's the same modem that works on the residential service (which is the same physical network). It won't help, just say "I bought a <VENDOR> <MODEL> modem, can you set it up on my account?" and hang up and try again if they act like your request is bizarre.