Just been looking at this in our Hub 3.
With "Smart WiFi" Enabled or Disabled ...
When I change the Security to WPA2-PSK for the 2.4Ghz band, the greyed out value for 5Ghz follows & also becomes WPA2-PSK.
A possible work around...
To gain access to the 5GHz security setting change the 5Ghz SSID just add an 'x' to the end of the SSID
Now adjust the 5Ghz security setting to WPA2-PSK
Then delete the 'x' from the end of the 5Ghz SSID.