IP address, DMZ, firewall - AT&T wireless basestation (MF279)

Is there any way to see a computer sitting behind my AT&T Wireless Internet device model MF279? I am assuming that AT&T assigns the dynamic IP so it would be a moving target. If I know what that moving target is at any one time, and assuming I've opened ports in the firewall, is there a way I can I "see" that device? I have seen some suggestions for putting a 2nd party router behind the wireless device, putting the internal IP for that device in the DMZ.