@David-TP 

Thanks for replying and giving me more debugging options. I think I have concluded that this is in fact an error with Amazon.

> IOT network+ no password+deactivating fast roaming

No Luck.

> If yes but still failed to connect, would there be any error message?

Using the mobile hotspot the error was:
"There was a problem setting up your device. Please try again."

Using the passwordless IOT network the error was:
"The password you entered was incorrect. Please try again."

The password message could be because that network is "saved" 

> Could you please help me test whether it could connect to a mobile hotspot?

No Luck. (This surprised me), so I just checked with my old nighthawk router and it won't connect to that anymore either, so this is probably an error with Amazon and the "deregistering" process.