While you could buy a Wi-Fi repeater to expand your home’s internet signal, an old router already has everything you need. There are various methods to do this.
One way is to connect your routers with an ethernet cable and switch the old one to AP mode. Another way is to log into your router’s admin settings and switch it to Repeater Mode.
Simply plug each computer into your old router with an ethernet cable. If you don’t have enough ethernet ports, you can always buy a network switch to add a few more.
You’ll need to log into the router to disable the DHCP server, change the IP to one that’s not being used, disable its wireless function, and connect to it with an ethernet cable.
If you don’t want to share access to your Wi-Fi network with guests but still want to be a good host, you can use an old router to create a guest Wi-Fi.
You’ll need to log into the old router as an admin on a web browser, enter the guest network settings, and click enable. Give it a name to distinguish it and set a password.