Expert with dual band router

[Unclemonty]

Well that sounds like a Wifi problem.  I get very variable internet connection speeds on Virgin (anything from 1-125 Mbps Wifi and 9-165 Mbps wired).
If you are streaming directly from your QNAP then I would investigate the Wifi connection.  Is it possible to check connection speeds and signal strengths?

Yes it does sound like a wifi problem, though the wifi signal is quite strong in my flat and the Devialet is in any case connected to the same router by Ethernet. Not really sure how to tackle it.

Thanks