In this case you will need to share one of the IP addresses you have (or the only one you have if you have a single IP DW4020) across as many computers as you need to using Network Address Translation (NAT). The preferred way to do this would be with a commercially available broadband router.
Attach the broadband router to one of the Ethernet ports of the DW4020. Following the instructions for your router, give it a static IP address in the range you wrote down for available IPs for use on your LAN. Enter the subnet mask and default gateway you got from the user interface. Enter the DNS addresses given to you by your ISP, or you can use 188.8.131.52 and 184.108.40.206. Set up the DHCP server on your router to give out addresses on the private sub-net you wish to use on your LAN. A typical range would be 192.168.1.2 192.168.1.254. Depending upon your router, this might be entered as an IP of 192.168.1.0, subnet 255.255.255.0.
Now attach the computers that you want to share the IP address to the router via Cat5 cables. Set each computers network properties to get its IP address and DNS automatically. After re-booting each computer, you should be able to browse the Internet.