I am not sure if I understand your question correctly.
If you are trying to publish an internal website with a different port then 80 over the MX to the internet with port 80, you can do it with portforwarding under firewall rules.
So point your external DNS to your MX IP and configure a portforwarding for your MX IP port 80 to your IIS with the specific port.
If you do it on the Meraki MX you can use port 80 only once, meaning you can have only one webserver / TCP port combination published with port 80.
If you have more then oine you need to use a reverse proxy or the like. The Meraki MX is not a reverse proxy.
Am I understanding your problem correctly?