DNS records need to point at LAN IP of the reverse proxy.
If proxy and destination are both running as containers on the same host, then place them together in the same Docker network and use the container name of the destination as hostname, and use the internal port of the service, not the port you might have mapped to the host.
Why not make this a lot easier and share exact details? NPM settings, Docker compose files, log output?
Btw /r/NginxProxyManager exists for this.