Wi-Fi: Guest network clients can communicate with each other, but cannot connect to the Internet
Computers and devices (Wi-Fi clients) that are connected to a Guest network may be able to communicate with each other, but cannot connect to the Internet.
This can happen if the AirPort base station providing the Guest network is configured to use a DNS server on the local network.
The base station must use an external DNS server, such as that provided by your ISP.
To change your DNS setting do the following:
Open AirPort Utility (in the Utilities folder).
Select your AirPort base station.
Click Edit.
Click the Internet tab.
Delete the manual IP addresses set for DNS Servers and IPv6 DNS Servers.
Click Update.
The AirPort base station will now use the external DNS server, and devices on the Guest network should be able to connect to the Internet.