@Hamaz 

Hello, you need use the main router's wifi password to configure this extender. 

Below is the instruction: 

https://www.tp-link.com/support/faq/698/ 

https://www.tp-link.com/support/download/tl-wa850re/#video

And what about the current hardware version of the extender? what is the led status as well?

Please log into the web UI of the TL-WA850RE and show us a screenshot of the status page and network page. 

Besides, what is the model number of your Zyxel router?