@isometric 

Hello, the wifi security and password of the extender actually are the same as the host wifi, can you please check again if you entered the correct password?

If the wireless password of your host wifi is correct, please tell me what are the LEDs status on the 850RE, you may also try to reset and reconfigure it as below:

How to configure my range extender via web GUI

Good days.

@isometric 

How about the LEDs on the WA850RE after the configuration?

Please follow the instruction to log into the web page and tell me the current hardware and firmware version.
https://www.tp-link.com/en/support/faq/1398/

If the WA850RE firmware is not the latest one, please update it first.

https://www.tp-link.com/en/support/faq/1416/

If all the LEDs are correctly on, please power cycle both your main router and the WA850RE, then login to its web GUI again and show us the full screenshot of the status page. And please export the system to us which you can find on the settings-system tools-system log and please tell me the model number of the main router.

Thanks in advance, and have a great day.

@rob.e250  Hello, 

If only PC work unstable with RE, it is recommended to update the wireless/ wired driver on the PC. 

May I know when did you purchase the range extender and how long the extender has been worked fine before? If it has been worked for a few years, you may consider upgrading to a dual-band wifi extender or extension products.