I just got this working. I just bought Google WiFi in hopes of improving the WiFi networking, but I have several devices on my network that still need to be connected via Ethernet. Devices like printers didn't show up on the same network as the WiFi when I just used the Archer C5 as a downstream router. This is basically the same idea as David's answer, but with a little fleshing out.
- Plug the ethernet cable from your cable modem into the main router's WAN port.
- Connect an ethernet cable from your main router's LAN port to your TP-Link Router's LAN (not WAN!) port.
- Using a device connected to another of the TP-Link Router's LAN ports, go to its configured IP address in a web browser (probably 192.168.0.1).
- Go to Advanced -> DHCP-> DHCP Settings page. Select Disable DHCP Server, and click Save button, but do NOT reboot the router.
Go to Advanced -> Network -> LAN and change the LAN IP to an IP address that is:
- different from the main router's IP address
- in the same subnet network as the root router
- outside of the DHCP range of the root router.
(For example, my Google WiFi app showed the Router LAN address as 192.168.86.1, and the DHCP Address Pool starting at 192.168.86.20 and ending at 192.168.86.250. So I chose 192.168.86.2 for my Archer C5.)
- Save the changes and restart the router. (You should be prompted to restart.)
After the restart, everything should be peachy keen. If, like me, you just want to rely on your root router to provide WiFi, be sure to disable WiFi on your TP-Link Router. If, instead, you want to use WDS Bridging, there are some additional instructions to follow at https://www.tp-link.com/us/faq-825.html, which is where I got some of the instructions above.
Remember that if you ever need to change your router's settings again, you'll go to the new IP address that you configured instead of the original one.