Maybe try to isolate the problems a bit and tackle one at a time. DDNS is only needed yo get to your router from the world wide web. Once you can resolve a name to your router's changing public ip address you can continue to think about port forwarding in openWRT. Once you can forward incoming traffic to a host behind your router, let's say port 80 to your nginx instance, then you can think about configuring nginx, let's say mapping to different running docker containers depending on the name in the url .... etc.