If you have an extra Wi-Fi router, you can use it as an external access point easily. So, instead of throwing away your old and unused router, turn it into a very useful device. 
Here are the basic settings that you need to configure to do so. Let's start!

Take a piece of tape and block the WAN port.

Connect your computer to the existing router using either Wi-Fi or network cable.

Run Command Prompt if you’re using a Windows computer, type in “ipconfig” and press the “Enter” button.

Note down the Default Gateway (IP Address) that you find.

Go to System Preferences if you’re using a Mac. Then go to Network to find out the router’s IP address.

Now, connect your old router to the computer to see what IP address it currently has.

Collect that IP address and open the router’s web interface with it.

Go to Network Setup part and then select LAN Setting and change the IP address to 192.168.1.2

Then turn off the DHCP Server and hit the “Apply” button to save the changes.

At last, connect the old router to the new router using a network cable and it will work as an external access point.

How to turn an old Wi-Fi router into an access point