Alas, there was no way to get the 2Wire to do this because, for reasons that are still unclear, unless the QNAP was getting its IP address from the 2Wire's DHCP service, it was effectively invisible to the 2Wire.

Moreover, the 2Wire lacks an address reservation facility and you can't set up port forwarding just using an IP address as the destination; the 2Wire only allows you to pick internal destination devices from a list it compiles using the gods-know-what mechanisms.

Why the QNAP, which is running all sorts of services, including CIFS, AFP, NFS, FTP, NTP, etc., as well as using the 2Wire as its primary DNS, wasn't "seen" while other servers (also using static IP addresses) were detected is still and will remain a mystery (unless one of you comes up with a good explanation).

Nope, in the end I decided that the 2Wire 2701HG-B DSL Gateway needed to be terminated (I almost went with "terminated with extreme prejudice" but I refrained … you never know when an old device is going to save your ***).

I decided to try another DSL modem I had lying around: A which, unlike the 2Wire, doesn't have built-in WiFi but, as I have plenty of WiFi access points hanging around, this isn't a problem.