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.
Best Answer
Well, as it turns out there IS a way to do this.
Connect the TC to the ISP's router using an ethernet cable. This will cause the ISP's router to allocate an IP address for it. Go into your ISP router's settings and find the place where you can reserve an IP. It will probably ask you for the IP and MAC address. You can find these details on the page where the router shows all connected devices.
Next, open the DHCP settings page on the ISP's router and reduce the allocated range. For eg: if the range allocated was from 192.168.1.1 to 192.168.1.253 change the ending address to 192.168.1.150. This is just an example, you can change the end address to anything you want.
After you have done this, go to your TC's internet settings and change the address allocation setting to Static; by default it would have been set to DHCP + NAT. Enter the IP address you reserved in the previous step, here. In all possibility it would already have be filled in.
Next, go to the DHCP page of the TC and change the allocation range to 192.168.1.151 - 192.168.1.250. We are allocating the remaining range to the TC so that it can hand it out to any connected clients.
Restart the ISP's router. Once that is up, restart the TC.