You need to connect 4 computers to the internet. Now you have 2 routers, but each with 3 LAN ports. How would you solve this issue?
There are 2 methods to solve this problem.
Use the second routers as a hub. This is easier, and you don’t need to configure anything, just wiring them up.
The second method is more sophistics. Look at the following picture. You need to configure a level-2 router. This means you need to set up a new local network on a local network, and these networks should be different. Then how to make them different? Consider the level-1 network (the first network) is 192.168.1.0, which means every computers connected with this router has an IP as 192.168.1.*. After you wiring up your 4th computer to the Router 2, you configure Router 2 through a web browser, and set the WAN IP as 192.168.1.10, (or 192.168.1.11, or 192.168.1.12… just make it different from the IPs of the first 3 computers, or you may choose DHCP to dynamically get an IP address.) Then set the LAN IP of Router 2 as 192.168.2.1. Then set the IP of your 4th computer as 192.168.2.2. It should work now.
Note the level-1 local network is 192.168.1.0, while the level-2 local network is 192.168.2.0. So they are on different local networks.