I am having the exact same issue, two brand new Era 300 that both fail to link to my account. I’ve tried pretty much everything, factory resets, etc.

I have other Sonos devices that I have had no issues with at all, same network etc.

I have found the solution if yours say “Not Registered” logout of the app. Then log back in go to the speakers and hit the fix it button. Should link immediately. That’s what I did and problem solved. 

Also had the same problem after my era 300’s stopped working as surrounds.  Reset both surrounds but couldn’t get them to link back to my account afterwords, so thanks for that solution.  Now I’m on to the next issue, getting them connected back into the surround sound system that for some reason they don’t want to join.  Sonos is a great system when it works, but every once in a while, it wants to try your patience by losing some connections for no apparent reason.  Not the first time I’ve had unexpected connection issues with some of my Era 300’s.

Thank You, this fixed my connectivity issues…