See my other post where I am using my Pi and reset my Wemo... Initially I was going the route to reboot the modem through the web interface until I noticed my stupid modem doesn't offer such option in the web interface! Since I had a spare Wemo Plug I was not using for a while I used that instead.

Connect a transistor to a gpio pin to drive a relay and power the modem through that.

Something I need to try... I haven't yet dig into all the wonders of the Pi gpio and that sounds like a fun project.