Guest account on AirPort
I use an AirPort Extreme for my WIFI access point, and I have a Guest account set up on it, but the guest account can't access the Internet. What must I do to make this work?
Time Capsule or AirPort-OTHER
The Guest Network feature will work correctly when the AirPort Extreme is set up to operate as the main DHCP and NAT router on your network. This would mean that you will need to use a simple modem with the AirPort Extreme. You can usually tell if you have a simple modem by looking at the back of the device. It will have only one Ethernet port.
As it stands now, your "modem" is likely a modem/router or gateway device that combines the functions of a modem and router in one package......and this would mean that the AirPort Extreme has not been set up to be your "main" router.....since it is likely configured to operate in Bridge Mode to work correctly with the modem/router. A modem/router or gateway will almost always have multiple Ethernet ports on the back of the device.
The Guest Network feature will not operate correctly when the AirPort Extreme is set up in Bridge Mode....and yet, that is the correct setting for the AirPort Extreme when it is used with a modem/router or gateway.
So, the first check you need to perform is verify what type of "modem" that you have. Does it have only one Ethernet port on the back of the device?
If you are not sure what type of "modem" that you have, post back with the manufacturer name and model number of the device and we'll see if we can identify exactly what you have.