Netgear EX6200 stops broadcasting SSID intermittently

I purchased a Netgear EX6200-100UKS AC1200 around 5 months ago.  I use it to extend both the 2.4GHz and 5GHz signals from my router: a Virgin Super HuB 2ac (provided by my broadband provider, Virgin Media).  

The issue is that the Netgear extender frequently (i.e. in the range of a few days to 2-3 weeks) stops broadcasting the 5GHz band.  This is depsite:

- the Virgin router still broadcasting its 5GHz SSID;

- the 5GHz status light on the Netgear extender showing green; and  

- the control panel on mywifiext.net showing the 5GHz band as still broadcasting and status ok (obviously I access mywifiext.net, in these circumstances, by connecting to the extender's 2.4GHz band).  

The situation is usually resolved by turning the extender on and off.  Occasionally (but much less often), this happens for both bands.  Sometimes, when I try to resolve the issue by checking the extender status at mywifiext.net, the SSID broadcasts again, but cannot be connected to (again, I need to turn the extender off and on in order to revert to my working settings).  

By the way, my router and extender broadcast different SSID names for both bands (this is how I know that the router is still broadcasting a working 5GHz band when the extender has stopped doing so).  Also, i have a number of different devices trying to scan / connect, so it is not an issue with devices not picking up the SSID. 

Any help / suggestions appreciated, thanks!  

p.s. I have a feeling that this issue started when I experimented with using the Netgear extender in "fast lane" mode (which I didn't stick to).