I just bought the same kit, also on Virgin media. When you say you have 2 ethernet cables plugged in, does that include the one to the powerline unit?.
I have one to the tp link, and another to a bt powerline box. The problem I had was with wifi, where they both refused to clone the wireless settings, so I eneded up with the vm router plus 2 tp link hotspots, and it was all a bit flaky. I downloaded the app from the tp-link website so that I could log into the units seperately, and change the ssid and pw to match the VM router ones, once rebooted they now show as a single ssid and seem to be working, except that I can no longer access the 5g wifi band.
The 2 ethernet ports on the units are in use and working fine which is good though.

Sorry, I just reread your post, you are saying that you can't have 2 ethernets in the adapter not the router...
As I mentioned, in one adapter I have a pc and a ps4, in the other I have a sky box and a smart tv, which are all working fine, so it sounds like you have a fault maybe?