ACS connection request via nginx

Right now all CPE´s are connecting to cwmp service via a nginx reverse proxy. Using upstream this works like a charm and during a session communication is via nginx. If the ACS is sending a connection request to the CPE this is done using the URL given from CPE and thus the request is not getting through the nginx.

Is these a way to overcome the asynchron communication?

This is more of a networking problem. Your CPE IP needs to be reachable from the ACS. If you can’t make that work then your only option is to use UDP connection requests (via STUN server) assuming your devices support that.