The NEST app on your phone is worthless. You can't add new nest devices to it. If you try, you are routed to google home to add it. The NEST app you can add via chromecast is also worthless. There is absolutely NO way to even sign into your account.
I would just throw the thermostat away, it's garbage.
Thanks for reaching out.
Have you added your thermostat to your account using the Nest app? If not yet, please follow the step by step guide here: https://bit.ly/3LElazT
Let me know how it goes.