I personally use a Synology. Isn’t there a "package centre, or “App-store” on the Qnap? I’ve installed the Mosquitto broker directly from the package centre. I don’t recall even configuring it (Don’t even know how to configure it 
) It works better than the Homey version. It seems to re-broadcast sensor data if a Mqtt device asks for it.
I believe you can also install it inside a docker container. (I have no experience with this).
