If you're getting indications that your network devices can't stay connected to the router (e.g. "cable disconnected"), then consider whether you have a factory-defective unit. With no configuration at all, these things work well -- right out of the box.
If you've just purchased this and it fails immediately like that, I'd consider marching it back to the store for an exchange. If the replacement also fails, then it's worth spending the time trying to figure out what it is about your network that is causing problems. Otherwise, you may spend a lot of time trying to configure something that was hardware defective.
That said, one quick thing to do -- use the reset button in the back.
1. Using a pen tip or similar, press and hold the button down.
2. Unplug power
3. Count slowly past 10 seconds
4. Restore power
5. Count slowly past 10 seconds
6. Release the button and wait for about a minute
Try to use the router now -- at factory defaults -- and you should not get disconnections during that trial. If you do, then consider going back to the store with it.
If your connection to the home network created by the DIR-655 is stable, but your Internet access is simply on-and-off, then reply back explaining what kind of Internet connection that you have and with details about your broadband modem and ISP. There probably are some configuration items to help you.