@julian5000 

Please give me the below details so that we can do more troubleshooting.

a. May I know the Model number of range extender. What's the light status of the RE  and main router when the Internet stops working/ buttering?

b. Try to give it a different network name/ Wi-Fi name from the main router so that we can verify the root cause.

c. Please connect your computer to the Range extender 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, you can unplug the range, and connect your smartphone to the main router by wireless to verify main router work stably?

e. Please ensure that the firmware is up to date. How often range extender got unstable issue? 

Thanks in advance, and have a good day.