Uncertain of what you mean by reserve IP address. I have not in the past reserved IP for any of my devices, and they continue to work, all except the Nexus tablet recently.
I just entered DHCP Server Setting, DHCP Reservation page, and entered the tablet manually as a client with a new assigned IP address. I saved everything, and rebooted my modem, router and tablet, and tried to connect to the internet. Nothing.
Any other suggestions?