I'm having an issue with SDP from external access, not on the LAN.

I've the server in my LAN and all users can access to it.

But I've made a reverse proxy with nginx to access from DMZ to the server. I've made a subdomain of my domain and redirected to the DMZ proxy reverse. That DMZ proxy also redirect to my LAN Server. I think is a good idea to not expose the server to the internet. 

But in this scenario I've issues that sometimes the server don't respond from the external URL.

I need help. 

Thanks,

Ricard Marti.