Does anyone know how "remote access" and the firewall interact on a Zyxel 660H

I'm trying to get remote management to work on my Zyxel 660H, without
much success at the moment.

Remote management is the set up to allow configuration of the 660H
from outside the LAN. It's simple enough to set the "Remote
management" menu to allow access from outside, it defaults to LAN but
you can toggle through LAN, WAN, All and None. So, I've set it to
"All" for the moment to get it going, and I see no difference at all,
I still can't get to the 660H from the outside world.

Everything else works as expected, the 660H connects me to the outside
world successfully and the port forwarding and firewall rules I have
set up allow me to get in to specific ports on my LAN from the outside
world but I can't get the remote management to work.

Does the remote management interact in any way with the firewall
and/or NAT routing? I.e. do I have to add firewall rules to allow
access to the remote management ports I'm using? If so what is the
destination IP address?