The problem appears to be related to the router in question. Unless I misunderstand the situation, it would appear that your router has appropriated for itself port 80 on each interface, probaly because it runs an embedded web server for the purpose of its own configuration and it cannot relinquish it by moving his web server to a different port or by limiting access to it only on its internal-facing interface.
That's usually some locked-in configuration of consumer routers.
A workaround might be enabling a ssl-only configuration on Zimbra, provided you router hasn't swallowd up port 443 as well.
From the shell you can give a
as root user or just
su - zimbra sh -c "/opt/zimbra/bin/zmtlsctl https"
as zimbra user.