If it is just a proxy server (no security software running on it) - then don't bother using it. Instead of setting up the NAT/PAT on the MX to the reverse proxy, point it directly at the server you want to give access to.
Yes, but unfortunately the customer is asking for a reverse proxy in a DMZ like in the picture i've shown above, and then the traffic would be forwarded to the backend web server
He already asked for a dns public registry of an URL xxx.xxxx.com
so i guess the idea is to map this url to the public IP address of the provider router and then port-forwarding the traffic on the MX towards the reverse proxy, am i missing something because that seems simple , what do you think ?