OK, got it working.
Sitting here at my office, miles from my DS212, listening to the music on my NAS through the DS Audio app on my iPhone. Got the video to serve through the DSFile App, and DSPhoto, at least to view. Haven't tried uploading anything from my phone.
I'm using DSM 3.2, just FYI.
So the solution (after EZ-Internet didn't work) was to go into the config of the (Cisco)LinkSys E4200 router, under 'Applications and Gaming' / 'Single Port Forwarding' and forward the following ports:
80 for PhotoStation
5005 for WebDAV
21 for Ftp
5000 and 5001 for AudioStation
All set to TCP, and forwarded to the fixed 'internal' IP address of the Synology NAS(already set this using the same router config app, under Basic SetUp / DHCP Server Setting (Enabled) / DHCP Reservation).
Used no-ip.com to set up DDNS when I did the EZ-Internet setup, and the URL I set up there now works.
Tip: Don't forget to enable the apps on the DS212, like PhotoStation and WebDAV and AudioStation.
>>>>THE PROBLEM WITH SYNOLOGY'S ROUTER SET-UP(?)<<<<
This router is actually listed on the dropdown of routers that EZ-Internet should configure. Unfortunately, in the listing, it says "version 1.0.01 Feb. 21, 2011". Well, mine has version 1.0.03, Sept. 2011.
And that, as we know, may be just enough to screw up whatever commands the NAS's s/w is sending to the router during the EZ-Internet setup.
So manually doing port forwarding on the router after doing the EZ-Internet on the Synology is the way to go.