Hi,
What is the current light status of the 855RE?
If it cannot detect the available wireless networks, first, move the extender into 3 meters away from the router, and please click rescan for few times and check whether it works; if still the same, please login to the web interface of the main router to change the wireless settings, especially the wireless channel, you can change it to channel 1, 6 or 11.
Besides, you need use the home wifi password configure it. Below is the instruction:
https://www.tp-link.com/support/download/tl-wa855re/#video
https://www.tp-link.com/support/faq/930/
May it help and have a nice day.