Your guys are suggested to report this problem to http://ticket.tp-link.com/index.php?/Tickets/Submit. To complain that you have the same problem here may be useless because we are all users and we...

When a Wi-Fi extender(A) is connectd to the router and PA4010(B) is connected to your PC, internet works? But if you exchange them, powerline light on B wil be off? So that you cannot access internet?

Tony provided the link of the instruction to pair two or more powerline adapers. Did you follow it?

Goood~ Share a screenshot here.

Doesn't support WPA/WPA2-Enterprise.

Operating system of your PC? Provide model number and its firmware version of your AC1750 router here. How about signal strength? Does it affect network stability?