I use to have this very modem and I think I speak for everyone when I say this thing sucks! However, there is hope. It sucks bad enough that I decided to crack it open and do some soldering onto it to get to a root shell. From there, I discovered a root escalation vulnerability in their web user interface.
What all this means? You can root your modem with a specially crafted HTML page, and configure it into a "true" bridge mode. In this bridge mode, literally the
ptm internet interface is short-cut through to an ethernet port, rather than going through the NVG510's router. This means the NVG510 can't even connect to the internet and will forward everything to one of the ethernet ports(which then goes to your router).. So, for instance, when you make a DHCP request for an IP, you get a response from AT&T's servers, not the NVG510.
I wrote a blog post explaining how to root it, and then enable true bridge mode on my website.
However, I must warn you. It's dangerous to do this and could possibly brick your modem. It's fairly difficult to brick the modem, but it's still a possibility. Also, it requires some experience using advanced things like telnet and familiarity with a command line.
Also, if my blog post is a bit too technical for you, or you don't have time to deal with it, I've made a $1 Android application called NVG510 Fixer. It fixes the most common problems (including bridge mode) just by the push of a button. No knowledge of telnet or HTML required.