By default port 443 is for the https, if you enable forward that port in your router, it will by default go to the web page to configure your NAS. So I wouldn't recommend forwarding that port as suggested by someone in this thread.
What do you need to do to enable Explicit TLS
Forward port 21 in your router, that's obvious.
What is not obvious is that you need to forward also data port. The problem it is a range, and it looks to be a big range. I haven't figureout the range yet, it would be nice if in the next release of the firmware this could be configure.
I don't know if Implicit SSL/TLS work, I don't have access to the router now, but the default are Port 990/TCP for the FTPS control channel and 989/TCP for the FTPS data channel.
Hope this help to figure out what happening.
I am also coming late to this party, but I have tried the Explicit setting on FileZilla and am met with
Directory listing unsuccessful (firmware 1.08). Is there anything special that I need to do to get this working?
I was unable to get connectivity using Implicit setting, do I need top forward port 989/990?