So my setup is as follows - everything is connected via ethernet cables.  The internet that arrives at the house is fast fibre broadband.  Firstly I have the standard BT modem/router that the system came with.  Then via ethernet cable I have a master deco (an M5), then I have a network switch, then ethernet cables to three satalite Deco M5s.

For the past few months the system randommly goes offline and all the decos show red lights.  Sometimes I can get it to resart by just powering each deco off/on, but this doesn't last. What seems to fix the problem for longer is setting up a new network name (username and password) but the network still goes down eventually. I have tried switching the main deco for another one.

I have tried all the troubleshooting steps suggested by the call centre.  You now suggest replacing the Deco M5s, but I don't understand how that will help.