Darren,
The AirPort Exteme (AE) is one of the older 'space ship" styles. Perhaps because it's older, this is the one I have the most challenges with. My setup is stable and robust but a few times this year (about once a quarter), the AE has gone off the grid. Power cycling it resolves the issue.

Since I wrote this article I've found it easier to set things up from the WDS main which for me is the TC. I would suggest reseting the AX to factory default ( you can find documentation here at Apple) and then enter the WDS Remote information into the WDS main. Make sure to write down the IDs of the AX before configuring.