Earl, Keith,
In my case, I tryed everything on that answer, but under Monitor -> Access Points I could see a External IP addr diferent from the one I set out manually. But since I knew which IP addrs the AP had, i connected via SSH and typed these commands:
set ipaddr l2tp dynamic
set ipaddr wan dynamic
I don't know if I needed to type them both, or just the one related to the interface that didn't "add up" (which, in my case, was WAN), but after both commands (2 auto reboots) the ZF discovered the ZD and upgraded the firmware in a couple of minutes.
This may be useful in the future.