This was exactly how my issue started, and it led to a whole lot of heartache. The case is still unresolved: https://www.googlenestcommunity.com/t5/Nest-Wifi/Request-Failed-When-Setting-Up-Router-After-Factory...
I suspect there’s a bug with the iOS app. Until there’s a fix, you may want to be careful not to reset or remove your wifi.
While I don't have news from our teams of a specific fix for the iOS bug that's being worked on, I do know that the issue has affected some people differently than others. Through troubleshooting efforts many have been able to work around the problem while others have chosen to wait for a solution to be found. From what you said, it looks like you haven't tried the network reset nor reinstalling the Home app, which are suggested steps for people dealing with this issue. If you are hesitant to try those steps, you are free to wait for more information later on. For now, we do recommend a network reset and reinstalling the Home app.