Set the address on the ATA to a static within your IP network subnet (subnet example 192.168.1.x). Make sure it is outside the DHCP range set in DD-WRT under the DHCP server settings. If the range is 192.168.1.100-192.168.1.140, set it to something like 192.168.1.150.

You want to set a static for the WAN address on the ATA, and plug that WAN port of the ATA into the LAN side of the DD-WRT router. You can then set up port forwarding(UDP 5000-65000) for 192.168.1.150, along with QoS.

Forwarding ports will be preferred to putting the device in the DMZ as it will reserve those ports for the ATA and no other PC or device on your network will be assigned the reserved ports.

I have SIP and RTP as Premium and Express respectively in my QoS settings. I can upload and download at full speed and not break up calls.