I'm trying to follow this.....but I am not clear on when the error message occurs. For now, please power off the AirPort Express and set it aside for now, and concentrate on the AirPort Extreme.
If the AirPort Extreme has been reset back to factory default settings again.....and
You have shut down the Mac for a minute, and then powered it back up.....and
An Ethernet cable connects from one of the LAN <--> ports on the HomeHub to the WAN "O" port on the AirPort Extreme.....and
You see AirPort Extreme under the heading of New AirPort Base Station when you click on the WiFi icon at the top of the screen.....

And.....you now click on AirPort Extreme.......
The first screen that should appear should look like this, except that you will see a picture of your AirPort Extreme model.

Does this screen appear?.......or......are you receiving an error message before the screen above can display?
If the error message displays before the screen above can appear......then that is a serious issue that may be very difficult to resolve.