Internet connection is fine (but not particularly high speed).
Those 210 files are missing from the archive - they return
404 Not Found
Can't assemble ubuntu-16.04.3-server-amd64.iso using jigdo
with 210 of the files missing (have the other 1264 files).
The Jigdo files specify the hashes, and
$ jigdo-lite
http://releases.ubuntu.com/16.04/ubuntu-16.04.3-server-amd64.jigdo
...
http://archive.ubuntu.com/ubuntu/
...
Aaargh - 210 files could not be downloaded.
Dang, ... really? 404 Not Found on 210 of the required files?
These are what's missing:
