Your "modem" may not allow the Guest Network feature to enabled if it is also performing as a router on the network.
Apple assumes that you are connecting a simple modem to the AirPort Extreme....not a modem/router device that many Internet Service Providers supply with their service.
What is the make and model number of your "modem" so we can check on that possibility?
I had the same issue, later to discover that the "guess network" is available on the wireless tab, not sure this will help on your case tho. Depends also on how you have your connections set, is the AE in bridge mode?
Bob, you mention that Apple assumes you have a simple router. I have a gateway router, SMCD3G with Comcast and a pool of 5 static IPs from them. Maybe I need to return the AE becuase I'm having issues trying to assign one of them to my Lion Server, like not being able to open ports if in "bridge mode", grayed out options. Can you shed some light on this? Appreciated.
I have a combo modem/router supplied by Xfinity (Comcast). The model seems to be Arrus TG862. From your reply I suspect that may be the problem. I had to get Comcast to install this new modem because the one I was using when I purchased the Extreme would not provide a stable/valid ip address to the Extreme. The previous modem had bee working fairly well with an old D Link router. There are many mysteries with wireless networking!!
Thanks for the info, Unfortunately, as suspected the Arris model that you have is known as a "gateway", or "modem/router".
The AirPort Extreme recognizes that there is another router "upstream" on the network, so it correctly chooses the Bride Mode setting to allow correct operation on the network.
The downside to this is that the Guest Network cannot be enabled when the AirPort is configured in Bridge Mode.
If the Guest Network feature is important to you, check with your service provider to see if they can supply a simple modem to connect to the AirPort Extreme.....not a "gateway" or "modem/router" device.
I upgraded my modem yesterday to a SMCD3GN gateway router (the only one offered with faster modem technology) and ran into the problem being described here of losing my Guest Network due to having to switch my AirPort Extreme to Bridge Mode. However, after speaking with my internet service provider, I learned of a simple solution. Change the gateway router to Bridge Mode (ie. disable the gateway/router functions to render it a simple modem - my ISP did this for me over my cable connection, or I could have gone into the settings myself to make the chg), and then change the AirPort Extreme Router Setting (in the Network tab) back to "DHCP and NAT". You should then be able to check the box to Enable Guest Network (in the Wireless tab).