Extending WiFi Coverage

Lots of ways to do this, but I'm going for the cheapest option. With a number of modem routers at my disposal, I've decided to link two of them using an Ethernet cable. If running a cable between the two routers is not possible, you can also go via the wireless route, but expect a throughput hit.

1. Firstly, make sure the primary modem router is working fine. No point setting everything up and not knowing where it went wrong.
2. Make note of its IP address and the IP range that its DHCP server is managing (usually 192.168.0.10 - 192.168.0.209).
3. Power on the secondary (modem) router and do a factory reset. This is usually done by pressing the reset button for 5-10 seconds.
4. Connect PC to the secondary modem. Access management GUI (usually 192.168.0.1) on a web browser and change its IP address to one within the primary router's DHCP range (192.168.0.200, for example). Take note of its MAC address.
5. Disable DHCP service (and NAT) of the secondary router, since the primary router will now handle that.
6. Create the same WiFi SSID (with the same password). Or even create a new one to differentiate the two routers.
7. Connect an Ethernet cable to the LAN (not WAN) ports of the routers.
8. On the primary router, assign a static DHCP lease to the secondary router (based on its MAC address) to minimize the chances of an IP address conflict.

Voila! Two WiFi routers linked by an Ethernet cable, providing extended seamless wireless coverage.