WA850RE and WA7210 are both running in latest firmware? WA7210 is in access point mode connecting to LAN port of W8961ND? 4. The AP & RE connected together and RE broadcasting the signal, but when I...

When did you buy it? W8901G is old and has been discontinued. Your guys have to buy new models.

Sean, maybe this instruction helps. http://www.tp-link.com/en/article/?faqid=508

Follow the guide as below to enable WLAN AutoConfig on your PC. http://www.tp-link.com/en/article/?faqid=306

Wi-Fi signal can be improved but the speed cannot. You didn't mention the Wi-Fi speed connecting to 300M router, did you? How did test or check the speed?

What is SID? SSID? Wi-Fi name? Wi-Fi password? On router page, you can delete the old Wi-Fi name/password and create a new one. Save the settings and reboot the router to make it take effect.