Can you play online video games when your PC is connected directly to the modem? You'd better have a test and try to figure out if it is a router issue.

How about upstream and downstream on status page of W9980? Can you provide those info on your HH5 as well? What's the current firmware version? Did you try to download something online and check the...

I'm afraid not. http://www.tp-link.com/en/article-15587.html

Your model is Archer T4U? Where did you get the driver for linux? As I know, it is not compatible with Linux.

Firmware of your WA854RE is up to date? How many signal lights when it works OK and it stops. What's the channel on your main router? Did you try to change it and reconfigure the range extender?

Connect a PC with gigabit port to LAN port of WR1043ND and check the connection speed, like the thread here. http://forum.tp-link.com/showthread.php?82682-TL-WR1043ND-v2-100-Mb-s-Ports