So I have two, paired, TP-Link TL-PA4020P Powerline adapters.

One is connected by Ethernet cable to the router.

The other is on another floor. The lights indicate it is part of a powerline network - three lights are illuminated top to bottom - ethernet, powerline and power.

I have a third TL-PA4020P that I want to join the powerline network.

I plug it in next to one of the other TLPA4020's and briefly press the 'pair' button on each. Within seconds, the lights stop blinking and the powerline lights illuminante on both.

I take the new adapter to a different location. Plug it into the mains socket, plug the Ethernet cable into it (the other end goes to a desktop pc with no WiFi). I get 3 lights again - but I can't get internet access.

Network Status shows as 'No internet access ' and under 'Network Connections'
the pc's Ethernet Adapter says 'Unidentified Network'

My old TL-WPA4220 adapter paired without a problem and connecting the desktop pc to that with the Ethernet cable and I get access to the internet.

Problem seems to be adding that third TL-PA4020P powerline adapter. Says it is paired but won't pass internet traffic.

Can you explain (a) what is going on and (b) how I can fix it?

I have NO idea how to find the firmware version of the adpters.

For reference, the two adapters that work fine are called 'PTA-04' and 'PTA-05'  The PA4020 adapter that is paired but won't pass internet traffic is 'PTA-03'.

PTA-04 and PTA-05 are hardware version 2.0, PTA-03 is hardware version UK/1.20  Could this be the issue?