Since the wifi crash, I pulled the power cord to reboot and the 5GHz device I was having issues with has come good (touch wood 
)
Now another device on the 2.4GHz wifi is experiencing the same issue, however even though it's not accessible (by it's GUI or ping from another device on network). I know it's still functioning (I can check this online, the device uploads our home electricity data)
However can ping the device using Openwrt cli.
It's strange as it affects devices in an unscripted basis.