Hello,
Connect the wired network to one of the 4 (or more) wired LAN ports on the second router. Make sure the secondary router is setup to be a wireless access point (no WDS or other extra junk, just a basic AP) with the same SSID (wifi network name) and key/password and security type (WPA2 personal) as the main router.
Thanks.
For more to go:-
Google Wifi Setup
Setting Up Google Wifi