@crownvn This is the forum where users can help each other. If you need support urgently from TP-LINK team, why not contact them directly? Did you try to install the driver manually instead of...

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?