I had this same issue but the other way around. When I went to dd-wrt from Merlin's firmware. What I did was, I unplugged the Asus router for 10 seconds and also pressed the reset button while off and then again when on (doubt doing both mattered but this is what I did). I then did the samething to my ISP's router/modem (which is bridged). Only then was I able to get the DHCP to work with dd-wrt.
I have the new Shaw "Wide Open" 150mbit plan which I had to trade in my old router/modem and given a new one last week. I don't like this new one I was given, its been a pain in the ass.