@HJB2020 

Please help to confirm the following information about your problem.

1. What is the current network diagram?

2. What is the led status of the Deco P9 when you lost connection? How do you make it back to work after that?

3. When the problem occurs, with a computer connected to the main Deco directly by an Ethernet cable, can you access the internet? Try to simplify the network diagram, in other words, 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. 

Below are some suggestions you can give it a go.

1. Relocate the Deco nodes.

2. Swap the locations of the Deco, setup one slave Deco to the main Deco.

May it help and have a nice day.