@Kemalyusron Hello, 

Please confirm your WA854RE is the on the latest firmware first: 
https://www.tp-link.com/support/download/tl-wa854re/#Firmware

You could try some suggestion here: 

> Try to turn off Advanced WiFi settings router like : Band Steering or Smart Connect; WiFi optimizing etc.

> If RE LEDs look normal when issue happen, please log in to Range extender's web page , turn on RE's DHCP and set up a static IP address for RE as FAQ below and see if that help with unstable issue:  https://www.tp-link.com/support/faq/2965/

Also make sure you have a stable Wi-Fi connection from the main router at RE's location.

If the issue persist, may I know the model of your router, and how about your network topology when did you buy this RE and when did the issue start, thank you.