The DHCP is generally set by default on most routers, but you can configure it by going to the Router's web user interface once connected through the ethernet cable with a computer. In your case, connect your laptop to one of your modem's host port, and enter the ip address of your router. If you are unsure, you can check the manual for the default IP address, normally it is either 192.168.1.1 or 192.168.0.1 and rarely 192.168.1.0. Once there, go to your basic router settings, and you would see the DHCP server settings, where you can enter a new IP address for your router and then set up how many DHCP IP's would you want. DHCP is basically automatic allotment of IP's on connected machines through the router.