Complex Proxying (NAT) with NGINX

Hi everyone,

I have a challenge I need to solve with NGINX. I have tried just about everything and i cant get this to fully work.

I have a front end facing NGINX server which has several sites hosted.
I need one site to proxy to another back-end server that is not on the same network but over MPLS networks somewhere and load that page in a "https://www.example.com/hiddensite/" Basically, that other back-end server is a full website with DB and reports. The back end server is running in a non encrypted HTTP protocol as its in-house and was build that way. Outsiders do not have access directly to the back-end server I'm trying to NAT Proxy with NGINX.

I have gotten the site homepage file loaded with NGINX proxy, but user cannot click links and browse the site at all.

Please help me if anyone know how to get this done properly.

[internet]---[firewall]---[nginx server]---[lan1]---[MPLS]---[lan2]---[hiddensite server]