said by mactalla_ :Sure, let's see if I can write coherent instructions here
Preparation: A-C
A. First, you need to be able to make changes to the routing. I'm not aware of any OEM firmware on routers that permit you to do this. This is (one) reason why many people like to replace the firmware on their router with something more powerful. The OP is already using DD-Wrt, which can do this easily. Myself, I use OpenWrt, so I know how to do it on there, and many people here use Tomato, which I have not tried, so would need someone else to confirm, but I'm 99% sure it'd work on there, too.
B. Now, you need to know the IP address your modem will respond to. The st516 will talk on at least one of its 2 pre-set IPs even in bridge mode and DHCP off. We'll call this $MODEM_IP
C. Finally, you need to know the name of the interface on your router where your modem is plugged in. (this can help: »
wiki.openwrt.org/OpenWrtDocs/Configuration ) We'll call this $WAN_IFACE
Steps: 1-3
1. So, now you need to set the interface identified in 'C' to an IP address that is on the same subdomain. For example, if the modem is on 192.168.1.254, then we want 192.168.1.pick-a-number (where pick-a-number is not 254 (the modem), or 255 (broadcast), or any other machine on the network. 250 would likely be good. We'll call this $WAN_IP. The command "ipconfig $WAN_IFACE $WAN_IP" should do the trick.
2. Now, we need need to update the routing with these two commands:
iptables -A forwarding_rule -d $MODEM_IP -j ACCEPT
iptables -t nat -A POSTROUTING -d $MODEM_IP -j MASQUERADE
3. And of course you want that to happen automatically anytime your router (re)boots. So make a script that does this and have it run on boot.
On DD-Wrt routers, I understand this is accomplished by creating files in /jffs/etc/config/ and chmod +x them.
On OpenWrt routers, the files are placed in /etc/init.d/ (and don't forget the chmod). Mine is prefixed S52 (I think -- will check at home) and the execution order works.
On Tomato routers, could someone chime in here?
I can paste my actual script when I'm back home.
That's an awesome guide thanks a lot! A lot of people will find this when google picks it up and make good use of it. 
