JTS wrote The thing is, there is already a BT extender unit in place and working, which feeds catch-up WiFi for my TV some distance from the Hub. So I was hoping the WPA281 would work with this as a...

Ethernet light on PA4010 is flashing? Any anti-virus software running on your PC? What is it? Try to disable it and run the utility again.

I don't think so. Some TP-LINK modems have this feature. You'd better report it here and confirm with TP-LINK technicians. http://ticket.tp-link.com/index.php?/Tickets/Submit

Your connection type is PPPoA, right? If so, when W8980 works in bridge mode, you have to select PPPoA instead of PPPoE as the connection type on Airport. Is it possible?

Report it here, http://www.tp-link.in/support/contact/.

Why you buy a PCI adatper to replace a USB one? Did you try to adjust antennas on WDN4800? Try chipmaker's driver, the one for AR9380. https://www.atheros.cz/atheros-wireless-drivers.php