I think you may have to connect them to a computer as I don't think the Extender Discovery Tool will work with an Extender in Standalone mode.

There are threads about how to use it and get it to work. You have first start the tool then flip switch on back (1-2-3 set to 3) to put extender into 'Configuration' mode. At that point, clicking on the extender line from the tool should launch a web browser where you can see an HTML page with all the information on it.

Again, it has been a long time since I used that tool and every extender I have is wired to my network.

If it is a single extender and any extender you exchange into that ID, you might try simply switching to a different extender ID. Also play with the placement of said extender as maybe there is RF interference around the extender and it is playing havoc with the wifi signal.

The RFX9400 has been known to require reboots, especially if not using the latest firmware (available in the files section).