Hey there Bencdavidson,
Sorry to hear about the loop when you're trying to sign in, I'm sure that is frustrating, but I'm happy to look into this for you.
If you completed the migration process already, you have to select "Sign in with Google" when logging in to the Nest App. Have you tried to log in using the web app? If not yet, please give it a try and let us know if the same issue persist. Here's how:
Let us know how it goes.