@Bijukk  

Thank you for your inquiry about tp-link range extender.

Please make sure your extender firmware is up to date first.  

Set up a different Wi-Fi name for extender ( to see if the router is on router or RE's wifi) and put it closer to router and test stability. 

Please refer to this FAQ to troubleshoot at first : https://www.tp-link.com/support/faq/2233/ 

If still the same issue, can you help us confirm some information:
1. What about the LED Status on RE when this issue happens?
2. May I know the problem occurs on 2.4G or 5G wifi?  How long do you have this RE and when did the issue start?
3. The firmware and hardware  version and model number of RE & Model number of router
https://www.tp-link.com/support/faq/315/