Problem with SDP from the outside

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.