Hello @mrussiello. The unique difference using the built-in STUN/TURN between OpenVidu CE and OpenVidu PRO is that relay candidates in CE connects directly with the media server because both are in the same machine. In OpenVidu PRO relay candidates uses the public IP as relay candidate because it is the sanest default configuration for every environment, but maybe in your case is not working.
I suggest you to add this environment variable in /opt/openvidu/.env:
COTURN_INTERNAL_RELAY=true
This will tell to the Coturn server to use the internal IP as relay candidate to connect to the Media Server.
Maybe this works, give it a try.
Regards