No linux driver for T4UH. Since you just got it, why not change it to a different model which may work with linux like TL-WDN4800...

No THX~ :)

Operating system? Try to update chipmaker's driver(AR9382). https://www.atheros.cz/atheros-wireless-drivers.html

Try chipmaker's driver( AR9380) here. https://www.atheros.cz/atheros-wireless-drivers.html

Firmware of your WA850RE is up to date? Since reboot doesn't help, how did you solve it normally?

WA801ND is working in AP mode? What's the firmware version of it? Is it up to date? How did you figure out that the problem is caused by WA801ND? Bypassing it, no problem at all? Provide the topology...