@Yakub 

Do you mean that the internet is unstable, it will lose connection now and then? 

If so, please give me the below details so that we can do more troubleshooting.

a. What's the light status of the RE450 and main router when the extender stops working?

b. How often it drops connection, and can you see the extender's wifi at that time?

c. Please connect your computer to the RE450 directly when the connections drop out. And please click here to check the IP address, default gateway and DNS server as the below instruction.
Note: Please show me the screenshot of the status-details page.

d. At that time, please connect your computer to the main router directly and test the IP, default gateway and DNS too.
Note: Please show me the screenshot of the status-details page.

e. How far away between the extender and main router And what's the wifi signal strength?

f. Besides, you can refer to the instruction below to locate the issue as well. 

https://www.tp-link.com/support/faq/2233/

Thanks in advance, and have a good day

@Yakub 

Thanks for your reply. 

It is suggested to contact the retailer or the local tech support to verify whether replacement is qualified. 

https://www.tp-link.com/support/contact-technical-support/#E-mail-Support

Note: Please provide the model number, hardware version, serial number and proof of purchase and the actual issue you have. 

Good day.