How to connect secondary wireless router to main router?

[slin100]

BigE is spot on. Note that some routers do not have an explicit AP (Access Point) mode. If yours doesn't, then the equivalent option is to disable the DHCP server in the secondary router.

I have a tip regarding BigE's first suggestion of plugging the secondary router's WAN port into one of the primary router's LAN ports. You can configure the primary router to put the secondary router into a DMZ. This will avoid double NAT-ing traffic between hosts sitting behind the secondary router and the Internet. I used to do this with an AT&T U-verse gateway and my own router. The Ethernet-connected TVs were hanging off the gateway while the rest of my computers were safely tucked behind my router.