Connection issues is so frustrating. If everything worked fine until recently I would log into your router and change the wifi channel on the 5ghz and see if you can get a connection as you have multiple wifi channels within your router and literally could just be interference.
5ghz is a faster frequency but for range 2.4ghz is the preferred one. I know what your saying about preferring the 5ghz but due to so many devices connecting wifi these days you could just be picking up interference from neighbours equipment. Try the channel change within the hub and see if this helps.
I am a virginmedia employee but all opinions are my own.
I am having the same problem can you arrange for an engineer to contact me please. I am also having WiFi security issues when logging in with iPhone. I have reset the superhub2 settings twice now but I keep getting security messages. I also get regular messages saying the password for 5g is incorrect. The helpline is not very helpful.
I seem to be having similar issues (coincidentally) since I changed my BB package to reduce the speed (and price) a little. I have had stable 2.4 and 5G connectivity for absolutely ages (years) but since receiving the new V6 TV box and downgrading the BB speed keep losing 5G (2.4G seems to be unaffected).
I can re-boot the router or disable and then enable 5G and it comes back for a while. Trouble is the V6 loses connectivity and the only way to get that back is a power off (at the wall) and back on.
One easy change is to separate the 2g and 5g channels by renaming them. It can confuse some wifi kit if they have the same name. A simple _2 and _5 to the end of each name could make all the difference.
Thanks - did that when I first got the router. Also, forgot to say that nothing has changed in our house for at least a year (new phones) apart from the service change and new V6 box. And the router has been de and re-powered since this problem surfaced about 4 weeks ago.