Static IP's are set via the MAC Address which can be found in many ways, if the device is connected, you can UnMasq MAC and check the Sys-Info Page under Status. Anyway, lets assume you have your MAC Address(s) ready.
Go to Services >> Services and under DHCP Server hit Add.
Fill in MAC Address >> Hostname (Doesn't HAVE to Match, but its good practice) >> IP Address and leave Client Lease Time blank (Blank = Forever)
DO NOT HIT ADD AGAIN, THIS WILL ADD BLANK ENTRY THAT WILL NEED TO BE DELETED.
Instead, scroll to bottom and hit Apply Settings, or Save if you have multiple entries, and hit Apply Settings when done.