I currently have the following setup, which works pretty well, at least in the lower half of the house.

Two TL-PA7010P PLC plugs, one with the fibre modem connected and the other with the router connected in an adjacent room. 

I have one PC connected to the router with an ethernet cable and other devices connected via WiFi.

The WiFi range is poor and so I wanted to extend the range of it and provide an ethernet connection to one of the bedrooms. I bought a TP-Link TL-WPA4220 PLV 600 Mbps thinking I could kill two birds with one stone

I plugged it in and paired it with one of the existing plugs (the one connected to the fibre modem) and the network LED goes green on the new device, however it goes red on the existing devices and there is no internet available from the new plug either via ethernet or its SSID.

I briefly had an SSID 'TP-Link' displaying but it had no internet access. This has now disappeared from the list of available WiFi hotspots in my home.

I don't know if I've purchased the wrong item (in which case if someone can suggest an alternative I'd be grateful), configured it wrong somehow or if it's a defective device.