What's the hardware version of your WN821N? Did you try the driver on TP-LINK website? http://www.tp-link.com/en/download/TL-WN821N_V4.html

The latest firmware of TD-W9980 has fixed the bug that sometimes Youview can't be played in UK. But as I know, the Sky broadband ADSL2+ service uses MER to restrict third party routers from its...

No worries.If the same thing happens, TP-LINK support team should further look into it.

From the screenshots, the modem has been identified and Internet IP address is obtained as well. Mayb it is a DNS issue. On DHCP---DHCP Settings page, try to change Primary DNS to 8.8.8.8 and Save.

Firmware version of your MR3420 is up to date? What's the model number of your USB dongle? If it is connected to your PC directly, does it work without disconnection issue? What't the DDNS provider?

When a Wi-Fi extender(A) is connectd to the router and PA4010(B) is connected to your PC, internet works? But if you exchange them, powerline light on B wil be off? So that you cannot access internet?