1) Port forward a different port for each domain thru the firewall to the web server for that domain. (Do this to test if the website is working thru firewall)
2) Put all the domain web pages on 1 Server, port forward thru the firewall to web server and let host headers do the work for you. (This is what I have used in the past)
3) Setup a reverse proxy that you port forward to thru the firewall and then let the reverse proxy do it's work. If you get this to work, then you could move your reverse proxy to the firewall if desired as NGINX is an option as well as traefik (traefik can be retreived by using the mimugmail repo addin)