Selfhosted

I still can’t add 3rd party software that isn’t in their App Store

That doesn't seem right... I think you can use their "Container Station" to make an LXD or Docker container with whatever you want. You may have to enable "advanced mode." https://www.qnap.com/en-us/how-to/tutorial/article/how-to-use-container-station-3

If I had to guess, the container station might be giving the docker container a new network/ip address, one that the NAS is not using so that port 443 works and doesn't conflict with the NAS. If you start the container station then inspect the container you might see how they do it, but macvlan is typically how you would configure it.

services:
  my-lan-service:
    image: nginx:latest
    container_name: lan_container
    # 1. Attach the service to the custom macvlan network
    networks:
      lan_network:
        ipv4_address: 192.168.1.200  # The dedicated LAN IP for this container
    # 2. Ports are exposed directly to the LAN; do NOT use the "ports" block
    restart: unless-stopped

networks:
  lan_network:
    driver: macvlan
    driver_opts:
      parent: eth0                  # Change to your host's physical network interface name
    ipam:
      config:
        - subnet: 192.168.1.0/24    # Matches your physical local network setup
          gateway: 192.168.1.1      # Your physical router IP

Yeah, I can do that. I just wanted to understand why it work sometimes but not others.

It's been a few minutes since I've run Caddy, and like I mentioned, I don't own a QNAP. So, I'm just spitballing.