VLAN Bridging WAN and a LAN port
From DD-WRT Wiki
For WRT54GS v2
If your ISP gives you multiple public IPs, and you want to hang a second router or other device on your main router with direct access to the WAN, fear not, it's easy:
Your original VLAN should look like this (among other lines) if you run the command nvram show |grep vlan
vlan0ports=1 2 3 4 5* vlan1ports=0 5
So the following commands move LAN port 1 to the WAN VLAN:
nvram set vlan0ports="2 3 4 5*" nvram set vlan1ports="0 1 5" nvram commit reboot
That's it. No firewall rules or bridging needs to be set up. Port 1 is now a direct WAN port, and your second router/AP/server can use one of your extra public IPs.