WAN --> MTU set to 1400

Not if the router has some special mechanism preventing the extender to extend its Wi-Fi. From your screenshot the WA850RE itself is definitely accessing internet as it says NTP and DNS success. 1....

1. Log into your router, find the wireless settings, change the wireless channel to 6. Save and reboot. 2. If step1 doesn't work, re-install the wireless adapter via Device Manager.

Single bar is not important..Getting 4 bars is good enough. What router are you using? Model : TD-W8980 is not a dual band router.

What happened when configured WPA2-PSK? It should work

If it still don't work, contact the retailer or support hotline for a replacement