@Kongjie 

Did you try to connect your phone to the home Wifi network or switch to mobile data then update the firmware , see if that makes any difference ?

  @Little_C My phone is usually connected to the home WiFi network. I tried switching to mobile data and then updating the firmware, but it still failed.

I tried resetting the plugs back to factory settings and re-adding them back into the app; during that step it seems as though it does a firmware update, but I suspect it doesn't, because a firmware update still shows as available.

Now another thing that is happening is that the plugs are going offline at random times.