You can avoid DD-WRT by using a router that, like many, supports "DHCP Reservations" a.k.a. static DHCP. Here, you tell the router the MAC address of the device (ATA) and pair that with a LAN IP address (outside your DHCP range). When the device does its DHCP request, the router gives that MAC address device the IP address you reserved.

Of course, you need either static IP or DHCP Reservation in order to do port forwarding.
I have a Grandstream ATA from VoIPo and no port forwarding or other special config. for that device which is on the LAN side of my router. Just plug and play. My router was a Linksys 11g, and is now a Cradlepoint MBR900. Neither had port forwarding for the ATA.