freeRadius Server with Dynamic IP address.

Hello Everybody: I just figure out how to solve the problem of NAS has a dynamic IP address(single client entry 0.0.0.0). But how about the radius Server is also behind a NAT which will get a Dynamic IP address?(Server and NAS communicate with each other through Internet)! How could I set the NAS's radius server IP adress option? The NAS i use is Compex WP543 and Netgear WG103, i dont think i could use a hostname or domain name to instead the IP address. Please advice me, thank you very much. Joey