Okay, you have a couple of options. Disable DHCP on the DDWRT router and leave it on, on the other one, connect them LAN to LAN. You'll need to ensure that the DDWRT router has the .1 IP address for the range and not the shiny new router, otherwise your devices will try hitting the other router to get to the internet (in general, default behavior is to look at x.x.x.1 for the gateway).
As far as I know that should work fine. Or you can connect them up WAN to LAN and run double NAT. Or just disable DHCP on the shiny new router and keep DHCP on, on the DDWRT router, which is what I would recommend.