Exclamation point on airport icon when connecting to wired base access point
I've got a WNDR4500 router connected by LAN to my main Netgear router and acting as a wired base access point (to extend my wifi network). Its network is seen by MacBook laptops but when I try to connect, I get an exclamation point over the airport wifi signal icon in the menubar. Have I got a setting wrong? I currently have it set not to act as a DHCP server; I think maybe that's what the MacBook hates. What is the correct way to set this up so that Apple devices can connect to it?
MacBook Air, OS X Mavericks (10.9.5)