There is a security certificate exchange that occurs at the start of the connection process which doesn't seem to work when the Airport Extreme is used as the Internet access point. When I switch to using my iPhone as a Personal Hotspot, it works fine. Any ideas on what the missing configuration might be?
Looks like Airport Extreme acts as an ftp proxy, see https://discussions.apple.com/thread/2047569 And to make use of this proxy for FTPS connections it has either to do SSL man-in-the-middle (which causes certificate check to fail) or to reject the SSL upgrade (e.g. AUTH TLS command). Another way would be to do only the login with TLS and switch then back to plain text (CCC command), then the proxy could maybe listen to PORT commands.
Further discussions see https://discussions.apple.com/message/21689470