PC Magazine published a few tips for people looking to buy their own cable modems and ditch the monthly rental fee. It also contains a reminder: Make sure you know whether your modem is just a modem, or a combination device that routes as well.
If your cable modem provides a router, and you’re not building a DMZ, you’re gaining nothing by running an external router, and could be creating issues, especially if the two devices are broadcasting on the same channel. You may wish to disable the built-in router in the modem so you can run your own modified, better-secured router, of course. And if you’re not careful, you can end up with a situation where you have two devices NATting into each other, which produces additional latency.
I’d never seen this kind of situation before, but it’s an honest mistake. But still, a mistake is a mistake and it could be causing you unneeded pain. If your wireless connection isn’t as strong as it could be and your performance is slower than your friend who’s using the same service, it could be that you have the same issue.