I had to downgrade my QNAP to 4.3.4.0675, quite some while ago now, to get it to work on Sooloos. Since then it has run without difficulty. Obviously, disable auto update for the QNAP firmware. All the latest (non Meridian) apps on the QNAP don't seem to mind the older firmware. And if you are anxious about nasties from the internet with the older firmware just get a Bitdefender Box to safeguard your network (including, Bitdefender claim, your IoT devices).

I forgot to add that if you have difficulty finding the firmware I may be able to help you.