JohnBR wrote

Thanks, Peter.

I have just done that; got it working alongside the new ones.

I have now discovered why the old ones (AV500) were not working. I had gone through all the procedures including the re-setting of the extender by using a pin, but failed to get them to work. Having searched the internet extensively, it appears that my mistake was to not hold the re-set button for long enough; I found a site that explained that it is necessary to hold the re-set button for about 10 seconds (other sites said one second!), and that seems to have solved the problem.

I was very pleasantly surprised to be able to get the old unit to work as it is several years old and, although I remember it working properly years ago, I thought it must by now be past its sell-by date. The fact that it still works would suggest that the TP-LINK's products are of good quality.

@JohnBR I also had an issue where I had receive a new router (Virgin Media Hub 3), and the WPS pairing function did not work for my AV500 TP Link powerline adapters.

After having tried all the obvious suggestions (move the router near to the AV500 adapter, resetting the adapter and trying again, etc.), I _turned off_ the base AV500 unit (the one that uses an ethernet cable in to the router) and then tried the WPS pairing again. Now it worked without problem. Magic. I could then turn on the base AV500 unit again, and my extended network was back again on the new wifi.

@JohnBR 

I am on iMac 10.13.6 High Sierra OS

I did have to press reset for 10 seconds with paper clip on one extender.

Do each extender one at a time

Press Pair button on base AV500 unit (the one that uses an ethernet cable in to the router), then Pair button on unit (bottom of extender). Wait a while to pair.

Open tpPLC Utility

Username, Password:
Admin
Admin

Check box:
Enable Wireless Powerline Extender Radio
Enable SSID Broadcast

SSID: Your network name
WEP Password:  Your network password