David,
Ok what I added in the past was a linksys router, and I configured it for PPPOE connection, the fixed IP address was assigned in the router and I used NAT on the A800 with private IP address from the linksys assigned in tenor and extip address was the the fixed the ip provided by ISP, I am pretty sure about this, as for the speedtouch there were 2 ways to set it up and I had it configured in a way to pass the connectivity over to the Linksys, basically it was acting as a regular modem with no IP addresses assigned on it.
email me if u need further assistance
Hwyking@comcast.net