@Buramu 

Thanks for your information, as for the unstable connection, the root cause varies; the actual network environment is the most important variation. 

We will do some troubleshooting and try to locate it and figure it out. 

1. Is it the current network diagram? Cable modem--main Deco ))((slave Deco 1 ))(( slave Deco 2? 

2. Do both wired connections and wireless connections lose connection at the same time? When the Deco P9 stops working, please connect your computer to the Deco by wired cable and verify whether you can access internet.

3. In the meantime, please connect your computer to the cable modem directly, bypass the Deco units and verify whether you can access internet. If it works well, you can connect it back to the main Deco by wired cable and open Deco app to clone the MAC address of this PC. 

4. Try to simplify the network diagram, in other words, please connect the main Deco to the modem only, with other slave Decos off and then take some time to test the performance. 

5. You can swap the locations of different Decos or change the slave Deco to the main Deco to give it a go.

https://www.tp-link.com/support/faq/2400/

Best regards.