Have you solved your problem yet? I think I have a similar issue with one of my 2 Google Speakers. One of them, after a couple of years of working, has decided it cannot connect to my home WiFi. To the best of my knowledge, there have been no changes to the system or password and, as I said, the other speaker continues to work. I even moved the working one to the room the non working one was in and it still worked.
