Hello, it is weird, theoretically, the powerline adapter won't affect the performance of the main router. 

With the adapters connected to the main router, what is the speed you can get from the main router's wifi? Is there any error message regarding to authentication failure ?

You can hard reset the powerline adapters and repair them first. And you can change another cable with better quality to connect the main unit to the router. 

How to reset it: With the powerline adapter plugged into the wall socket, press and hold its Pair button for more than 6 seconds until the Power LED goes off momentarily and comes back on.

Besides, what is the model number of the router? 

Hi Anton L.Z.

Thanks for the response.

The model number of my device (printed on the device label) is TL-PA7020P(UK)

When the powerplug is connected to the router the wifi access speeds on other assets literally half to their normal access speeds. Dont have an accurate measurement, just observations. Whereas normally a particular website (in my quick test www.bbc.co.uk/news) usually opens in under 10 seconds. With the powerplug connected this can take over a minute to load.

I did have a quick google around and found customers of other powerplug adapters mentioning the same issue.

I think I'll return my kit to Amazon as this solution (powerplugs)doesnt appear to be what I am looking for.

Thanks for the time you have taken to respond to my post though. It is appreciated :-)

barneybloggs

Hey Barney, it is good to share something with you guys. 

While it is a pity that the powerline adapter did not work as expected. 

PS: you can try to update the firmware of the powerline adapters if you haven't returned it back to Amazon. 

Good day. 

You need to update the Powerline Adapter Connected cause most of the time it worked for me

@barneybloggs 

I have the exact problem with you and mine maybe worst and weird. Using Powerline WPA 8630P.

When connected to my main router, device is connected. But if surfing, i can only use it for 2,3 seconds then no data will be transferred. If i do a speedtest, it will go to 100mb for 2second but decrease until 0, cant even complete the test. Howerver,every devices connected to the powerline at my second floor is working fine. Only happen to devices connected to the main router. 

If I unplug the powerline main kit besides the main router,everything went ok. As soon as plug it back, i cant even surf internet. Any suggestion?

Is this because the signal from the adapter clashing with main router signal?