Yes, Enable Default Host in NAT is DMZ.

I don't know why you're having those problems. Something is awry. What is the external IP address that the Extreme is grabbing? Just post the first number, ex 74.x.x.x. I'm looking to see if your modem is also doing NAT, many of the new modems do that now, adding another layer of complication to port mapping.

I have not liked the airport bases until the last couple of years or so. They are up to par now, and have great range.