We have a wireless n Dlink 615 router connected to a cable modem.
What hardware and firmware revision are you at? To find out, go to the router's login page through your preferred browser like the one shown below:
Note the image above. Ignore the red text, that was for someone else who had the same router as I do, but different revision.
Note, in the upper right corner of the page, you'll see:
Hardware Version: B2 Firmware Version: 2.24
Let me know what rev you've got, and I can provide you further help, as I've worked on several different revs of this router.
Right now, I'm using Rev. B2.
The reason I'm asking is because it'd be easier for me to post picture guides so that you can fix it yourself. Or I can do remote desktop support and help you out that way too. (Though... I hope I'm not breaking any rules by saying that. I DO own a tech support company that uses remote desktop software to offer support to those who want to watch and learn. But that's an aside.)