Old subject of systems not registering with the annoying “check your network” message. There is a hint in the “Check the time and date on the unit” I found out by eliminating all the working SONOS speakers and leaving just the unregistered one, I had to sign in, but finally got past all the Registering failure. I went to system and checked TIME and DATE and it said Jan 1 2001 and 12:00 (The default). I tried to make it update to Internet Time so it would be accurate but it never updates.
My guess is the clock in the speak has a Lithium button battery that has gone dead after 8 years. And Registration can’t happen if the time is not accurate for some crazy reason. CS support is useless. This guy blamed my Internet provider which is the old ruse of years ago. Now with IPV5 and IPV6 it’s so standard that excuse is like saying it’s your 110V power outlet. Net Net, the clock is dead and the unit will never register. I’m going to take mine apart and see if I can update the battery. It’s a boat anchor anyway.