Just to recap the issue.

Upgraded to 1gig broadband with Virgin Media (UK) and Superhub 4 modem / router. All wireless devices worked fine including my desktop PC with onboard wifi

in both 2.4g and 5 g modes. Both connections totally stable.

Switched SH4 to modem mode and installed Archer AX6000 router. All wifi devices (iphones, ipads, smart TV, Alexas, Sonos speaker) worked fine but the PC either wouldn't list

the 5g signal or would see it / connect and drop out minutes later and would never reconnect to 5g on reboot. 2.4g mode was fine.

Purchased an AX3000 PCIe card (Archer TX50E - another £55 !) and installed it in my PC. WiFi signal on my PC now working perfectly in both 2.4g and 5g modes.

Just for completeness, the original PC wifi was built in to the Asus Prime-X299 deluxe mobo : Qualcomm Atheros QCA61x4A wireless network adaptor with latest driver.