@MrLorenz 

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

a. What's the light status of the TL-WA850RE and main router when the extender stops working?

b. Please connect your computer to the TL-WA850RE 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.

c. At that time, you can unplug the TL-WA850RE, and connect your smart phone to the main router by wireless to verify whether it works well.

d. Please ensure that the firmware is up to date. 

Thanks in advance, and have a good day.

@Kevin_Z 

In this month's I tried to do some test.

I have changed the work mode of the TP to access point, but the result is the same. 

The WiFi generated by TP is very slow compared to the WiFi generated by the main router (Fastweb)

In attached some photos of the speed test generated in the same time, first attached at the WiFi of TP and second to the WiFi of main router. Also other photos of the TP configurations.

@MrLorenz 

Thanks for your information. 

In access point mode, the TL-WA850RE need connect to the main router by wired cable. Try to change another test website, like Speedtest^® .

Besides, please ensure that the firmware is up to date. 

If possible, you can add the extender into high priority device list on router.