You guys won't believe it - now that I was successful in loading Merlin build, I got a little more brave... After hearing you and jsmiddleton suggesting Shibby build for setting up multiple-IP, I downloaded Shibby AIO-64k build firmware for my RT-N66U. I also saw in their tutorial in how to setup multiple static IP - WHOAAA - in less than 10 minutes my RT-N66U was up and running with my block of static IP from AT&T Uverse (I don't recommend that AT&T service but currently I have no other choice in my area, it drops a lot).
Merlin - it was extremely simple so perhaps even your build could do it? (just an idea)... All I had to do was:
1. Download/install Shibby's TomatoUSB firmware ending with AIO-64k for RT-N66U
2. Setup my AT&T router for "IP-PASSTHROUGH", and connect RT-N66U router in Router-behind-router config (Internet <--> ATT Router <--> RT-N66U router <--> LAN)
3. Enable JFFS (it goes through formatting JFFS partition)
4. Then in Shibby build, I had to go to Administration->Scripts->Firewall and enter the following lines for each of my static IP (seems that it automatically builds the script file for you):
/usr/sbin/ip addr add 99.XXX.XXX.105/30 dev vlan1
/usr/sbin/iptables -t nat -I PREROUTING -d 99.XXX.XXX.105 -j DNAT --to-destination 192.168.2.20
/usr/sbin/iptables -t nat -I POSTROUTING 1 -p all -s 192.168.2.20 -j SNAT --to 99.XXX.XXX.105
Where 99.XXX.XXX.105 = one of the static IP addressese from the block of 5 static IP assigned by AT&T, and 192.168.2.20 = LAN IP of my computer to which I wanted the traffic routed. I then repeated above three lines for three more static IP's and left one free (assigned to RT-N66U router so that all other computers can go through that)
After a reboot, it worked like a charm! I haven't tried adding such custom script to your build under /jffs/scripts/nat-start but perhaps someone who wants to experiment could try to see if that would work!
Anyway guys, I'm very happy that I can use my RT-N66U router for multiple static IP from AT&T. Hope this helps others who are looking for a solution too! Thanks to you all for such amazing one-on-one support.