2.4g wifi keeps dropping off

As per title.

Have a hub5, and has been working OK up until  a few weeks ago, when the 2.4g signal dissapeared.

5g was still connected, but 2.4g was showing as connected, even though the 2 pc's and 2 mobiles were showing 'no internet'  and were unable to locate the signal, even though the ssid was a prefered one. 

2 mobiles and a pc on 5G were all working and had full signal. 

According to the hub 5, all the 2.4g  devices were connected, even though they were not. 

Tried to connect the 2.4g manually, but all refused to find the SSID.

Ended up doing a reset on the router, (pull plug for 5 mins) then when the router restarted, after a few minutes, all was working OK again on both 2.4g & 5g. 

Strangely, it's happened again twice over the last 2 weeks, exactly the same. Wired not affected, but the 2.4g wifi again 'went missing' even though the hub said it was connected OK. yet again, 4devices on 2.4g lost their connection, yet hub insisted they were connected! 

Each time, a reset had to be done to get the signal back. Once reset, all works again for a while. 

Not sure if hub is on the way out, as strange that 4 devices should all lose signal at the same time, yet hub says it's working correctly.