Daniel, thanks. The IP address is assigned by the router–although as has just been pointed out to me in an email, the one displayed in Synology Assistant is wrong.
I’m about to mark the problem “solved”, however, and the solution is weird: I’ve long distrusted the cheap-assed Ethernet cable that came with the box; I just replaced it with one of good quality. And when I did, my NAS showed up.
Thanks
Jim