@Tarcia 

Hello, sorry for the inconvenience caused here; we will do further analysis to locate this issue and try to fix it.

Please give me the below details first.

a. What's the light status of the 850RE and main router when the extender stops working?

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

c. Please connect your computer to the 850RE directly when the connections drop out. And please check the IP address, default gateway and DNS server as the below instruction.
https://www.tp-link.com/support/faq/838/
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? Please ensure that the firmware of TL-WA850RE is the latest one.

f. What is the model number of the main router? 

Thanks in advance, and have a good day.