Quick update to this problem.

Appeared onsite to help a new customer and to my horror found one of these offending devices (Tenda w300d). I was going to suggest the unit be binned, but decided to give it a far go first.

as above I was able to complete dns lookups on the wan side. So updated with the latest NZ/AU firmware (http://www.tenda.cn/tendacn/DownLoads/show.aspx?downid=1649) and it seems to have closed port 53 and 23 on the wan side.

The customer has also reported an "increase in internet speed" and a drop in their consumed data according to their ISP's website in the last week or so.

Not sure what else this update fixes, cant find any release notes ?!?!

Hope this helps.