Hi WiltshireL, I'm and end-use like you. I'm not sure if this applies to you but I have seen a number of people not following the proper FACTORY RESET procedure. Here is a link to the proper instructions. Be sure to scroll way down to the section called "Before you factory reset" and then view the directions for "Nest Doorbell (wired)".
I hope this helps get it to the point where it can be re-added to your wifi.
Just checking in to see if you still need assistance with this issue. We'll leave this topic open for another 24 hrs in case there are any follow up questions or comments. Please let me know if you need further assistance.