SSH on port 443

[maxrussell]

I've set my sshd config to use port 443. I've also set my ISP firewall and router firewall to accept port 443. I can run

ssh -p 443 localhost

and log in. However, when I try and run it using my dyndyns name, the shell hangs/no response is returned to the command.

I've been googling and I've been hacking. What am I missing please?

EDIT - sorry, had multiple tabs open and posted this in the wrong place, could a mod please move it to a more appropriate section if deemed necessary?