How do I add a computer when the ip and dns address do not match?

I am using ARD to connect to a few computers at a remote site. The site has a Static IP, as well as each machine on the local network. One of the machines works fine (a Mac Mini running ARD 3.6), but that was set up a while ago, and any other machines (running ARD 3.6.2) I try to add will not work.

Basically, I want to add a machine with an IP address of the location (ex. 50.50.120.181), and the DNS name of the computer on the local network (ex. 192-168-1-21.lightspeed.stlsmo.sbcglobal.net). This is how the Mac Mini is set up and works. However, every time I try to change the DNS name, it changes the IP address to match (192.168.1.21). Since I'm connecting remotely, that doesn't work. If I try to change back the IP address, it just reverts it right back.

How do I get ARD to stick with the IP address I tell it to??