@BRM 

Hello，

Please tell me the LED light status on the TL-WA850RE.

According to the LED light status, then refer to this FAQ to do troubleshooting:

https://www.tp-link.com/en/support/faq/2234/

Note: When checking the router's wifi password, please also confirm that the authentication type and encryption type is Auto on the main router.

If it still not working, please tell me what step you have troubled, and what troubleshooting you have done, thank you!

Please also send me a screenshot of the error information, if it still has the same issue.

May it help nad have a nice day!