I would do this on my own '655; but, I have it running in AP-Only mode. So, I turn to the online DIR-655 emulator
Network > Network settings. Scroll down to 'add DHCP reservation.'
Check the 'enabled' box, and enter the computer name (its network name), desired IP address (IIRC, it must fall within the router's DHCP range), and MAC address for the computer in question, and hit Save.
Since the server is set up to get an IP address automatically, and DHCP rules state that a client will first try to request the same IP that it previously had, a reboot probably won't give it the new IP.
You'll likely need to run a Repair on the server's network connection (do this logged in locally - not via RDP), or an ip reset (netsh int ip reset, from a command prompt.)