I assign static addresses to my computers through the router interface, not through Windows, because I have had issues with DHCP conflicts and "There is an IP address conflict on your network" in Windows. I am using a DIR-605 (only available in China), which is similar to the DIR-615 except it uses a Broadcom chip.
I would set the computers to automatically obtain IP addresses via DHCP, and then configure the router to send static IP addresses automatically whenever it detects the MAC addresses of the computers that you specify.