TD-W8968 is the main router and normally repeater should use the same SSID and password. If it cannot, you should check the linksys device and its firmware.

Is it possible for you to install the WN823N on a different PC to test if the problem continues? Have you tried to update the driver? https://wikidevi.com/wiki/TP-LINK_TL-WN823N_v1

http://www.diffen.com/difference/USB_2.0_vs_USB_3.0

The firmware version is so old... Why not upgrade the latest firmware? http://www.tp-link.com/en/download/TL-WR841N_V9.html#Firmware Upgrade the latest firmware, reset and reconfigure it to test if...

WAN IP is private or public? If it is private IP, you have to open corresponding ports on the modem for router's WAN IP.

Upgrade TL-R470T+_V4_tmp first and then upgrade the TL-R470T+_V4_141208 one. You can read Note.txt in firmware folder.