An update! I have now got the AX connected to my network & music is streaming to it. I'm not 100% sure what was the key trick but here a list of things that may help!
1) I'm not using the Windows Firewall so I copied the entries for iTunes & the Airport Utility into my CA Firewall.
2) The Netgear wireless 'card' on my PC has it's own utility for managing the networks. So, I connected my pc to the router by Cat-5, so I could still see the router config and connected to the AX in the Netgear utility, having scanned for available networks. After this, the AX appeared as a 'Base Station' in the Airport utility list.
3) now I was able to get into the configuration of the AX & set it up for connecting to a network as a client. I had to set my router to broadcast it's SSID so the AX could see it, however. The existing WPA key worked fine.
4) The 'holy grail' of the steady green light appeared!!
5) After this I updated the AX Firmware, then moved the AX into the other room, plugged it in & success, the green light appeared again!
So, not quite as PnP as advertised, but I got there. Now to look for solutions to the short drop-outs that are occurring, must be some streaming settings somewhere or maybe the 'optical' connection may help?
Good luck.
Tony.