I have set up an AirPort Express Base Station (June 2012 model, the current one). For setup, I used a Yosemite MacBook Pro over Ethernet. The AirPort Express has an Ethernet running from the WAN port to a CLEAR modem (a WiMAX service) in "Bridged" mode.
Both the 2.4 GHz and 5 GHz networks work well, devices connect with password and surf the web.
The third network, the Guest network, does not let any client connect. I've tried both the MacBook Pro, an iPhone 5, and a MacBook Air repeatedly, power cycled, and so forth. When they attempt to join the password is accepted but the spinning wheel never stops and the computer/device never finishes joining the network. We can switch back to either the 2.4 or 5 network instantly with no problems.
Any suggestions? Might Bridged mode be a problem with a Guest account?