I just spent the last two weeks trying to resolve why my Zigbee stick (Sonoff dongle) was disconnecting and, having found the answer I thought I’d mention it here. I went through all manner of things, trying different firmware versions and playing with various parameters, all to no avail.
What turned out to be the problem was radio interference on the USB extension cable. I have my Zigbee dongle on a 3m extension cable and it turns out my wife had moved her Qi phone charger (a significant radio emitter) to the other side of the credenza and it was now within 4 inches of the extension cable. That was the rub. Moving the phone charger away and routing the extension cable more carefully solved my issues.
Similarly, if you have no extension cable at all, you may well be getting interference from the USB port itself.
So just be aware that interference on the USB line is a thing and route cables accordingly.