Hi Rich
Join the club.
I have exactly the problem which you have desribed and have spent many hours trying to fix it with, as yet, no success.
Interestingly with my one, I also get the same symptom when you set it up in its natural mode as a broadband router rather than using the WAN/DMZ port to another gateway (although I too need to use the configuration you describe). Wonder if you have tried this yourself and got a different result?
The scant documentation and other post on this forum do say that this configuration is possible but as yet no fix for this symptom.
Wonder if anyone from BT has any useful advice??
Will keep you posted if any resolultion is found, please reciprocate:)
Matt
