Please review what I said originally.......that the guest network function on the AirPort Extreme is designed to work with a simple modem......so the AirPort acts as the main router for the network..
Another way of saying the same thing is that the AirPort needs to be "in charge" of your network for the guest feature to work correctly. The AirPort cannot be in charge if it is connected to another device that is already configured to be the main router on your network.....your Actiontec modem/router.
The Actiontec device combines the functions of a separate modem and a separate router in one package. This type of device is known as a modem/router, or also known as a gateway.
Some folks call a modem/router or a gateway......a modem. So, things can get confusing.
I do not know if it is possible for the Actiontec device that you have to be configured to act as a simple modem.....so the routing functions of the device are completely turned off. (Turning off the wireless on the Actiotec does not turn off the routing function).
If you turn off the wireless on the Actionec, it becomes a modem and a wired router. And that wired router is still in charge of your network.
The guest network feature will not work correctly unless the AirPort is in charge of your network.
My suggestion was for you to ask your Internet Service Provider (ISP), if they could supply you with a simple modem. That is all that you need. You don't need two routers....and the Actiontec that you have now is not allowing the guest feature to work correctly.