Thanks for your replies! You were right, it was completely a router issue. The Ubuntu box was configured correctly.
I checked the router logs, and noticed an unexpected entry - "Administrator login fail, Access deny". It turned out that the router remote management port was set to 8080. This was an old setting that someone else set years ago.
I disabled remote access, now everything works great! It was such a simple thing - once I found the problem. I imagine you were just as frustrated when you realized you hadn't applied changes

. DD-WRT does have a weird way of saving changes.
Thanks again,
Devin