@Aki9308
Hello, we will do some troubleshooting first so that we can locate the issue and try to fix it later.
1. What is the current network diagram?
2. What is the led status of the Deco M5 when you lost connection? How do you make it back to work after that?
3. When Deco M5 stops working, with computer connected to the main Deco by wired cable directly, can you access internet? Try to simplify the network diagram, in other word, connect the main Deco to the modem only and then connect your computer to the main Deco to verify whether it works well.
4. If still the same, please connect the computer to the modem, bypass the main Deco to test whether the network works.
5. Besides, you can try to relocate them and swap the locations of Deco units to give it a go.
https://www.tp-link.com/support/faq/2400/
Thanks in advance, and have a nice day.