I think i understand what you want to do.
I have no idea how much current this device draws. That is an important consideration that you should understand to know if this will work for you.
Essentially what you’re wanting to do should work. Setting an output pin to TRUE will apply power to the device that it is connected to. If your device will draw more power than the pin can source (which is most probable !) then you will want to use a transistor to allow you to power the device - you switch the transistor and it switches the device.
The same applies to the device’s output when connected to your Fez, to an input pin. Check if the pin is enabled, and then you will find out if the device is powered on or not.
But remember, this is a reasonably complex thing to do and you risk damaging either the modem or the Fez, or both, so you really should know what you’re doing or get proper assistance if you’re not confident.