Are you using a bridged modem? What modem is it? Does your ISP allow pppoe connection?
In Europe many ISP use only PPPoA or something rather more complicated IPoA etc.
I would simply use the TC in bridge not the modem...
So go back to modem running in full router mode..
In the setup utility select network.. under network select off bridge mode.. there are posts with pictures but I don't use it so don't have it available sorry.
Post back if you cannot find them.