The DW4020 is available with several different service levels that provide from 1 to 13 unique static IP addresses. The first thing you need to know is the one address that is assigned to the DW4020 itself. You should have gotten this address at the time of installation. If you didnt, then the easiest way to get it is to plug a computer into one of the Ethernet ports, and set up that computer to get its IP address automatically. Reboot that computer. When the computer is back up, open the browser and enter "http://192.168.0.1" as the URL, without the quotes. This should open the DW4020 user interface. You should see a Welcome message. In the text of that welcome is a click here link. Click on that link. That will take you to a screen that gives you all of the IP addressing information you need to set up your network. Copy down the range of IP addresses that you can use on your network along with the subnet mask and default gateway. The address listed as default gateway is the address of the DW4020 itself. This is the address you will use in the URL of your browser from now on to access the user interface.
The DW4020 does not have a DHCP server to hand out the WAN addresses for your computers to use. It will only give one address to one device for the purpose of initially getting into the user interface as you just did. This requires that you set up each computer with a static IP. Start with the computer you just connected to the user interface with.
Open the network properties for the computer and enter a static IP in the same range as what you just copied down from the user interface of IP addresses to use on your network. Enter the assigned subnet mask, default gateway you copied from the user interface and the DNS address given to you by your ISP. If you dont know the DNS address you can use 188.8.131.52 and 184.108.40.206. Reboot your computer. Now you should be able to browse the Internet from that computer and access the DW4020 user interface by entering the same address you used as default gateway as the URL in your browser.
Do the same for each computer on your network, each time using a different IP address from the same range you wrote down earlier. The default gateway and DNS addresses will be the same on all the computers.