I would try letting the Apple Extreme handle the PPPoE login (ie bridge mode), just clear out all the login settings on the Sagecome and turn off its Wi-Fi as well. By doing so you can configure the DNS servers of your choice in the Apple Extreme, and Bell's hardware will not be intercepting and mangling DNS requests.
Well this leads to another can of worms ... maybe. I want my Apple Extreme to handle all the PPPoE connections but to do this, I have to put the Sagecom is bridge mode ... do I not? And Bell does not want to hear about it. They told me to call Sagecom but they won't support it. I am also concerned about screwing my TV connection when I do this? You are saying that if I simply remove the PPPoE connection information from the Sagecom and put it in the Extreme, that will work so that it is the Extreme handling the connection and the DHCP clients? Will that botch the TV signal?