I have a similar problem, but more information...
I have a nest gen 3 learning thermostat. It has been installed for a while and working fine for several years. I had a furnace failure and had the entire HVAC unit replaced. The blower motor locked up and due to the age, we just decided to replace the entire system. While starting up the new system, The technician was having me turn on and off the heat and AC functions to test the unit. The thermostat was showing offline. So, I went to the physical thermostat, everything appeared appeared to be fine. I could turn on and off the heat and AC by adjusting the setting. Also, the unit came online on the app. I thought everything was fine and just had to due with the system replacement. Approximately 8 hours later I went back into the app. The thermostat was showing offline for 30 mins. Everything appeared ok at the thermostat. I manually adjusted the thermostat, like I had done before. This time, it didn't come back online. I have reset the thermostat, I have reset the account connection, I have reset the the thermostat to factor defaults. I have deleted the thermostat off the app. I have reset my wifi, I have reset my phone. Every time when trying to add the product to my nest app (either by scanning the QR code or manually typing in the code) it tries to connect to the device to assist with the setup and it fails . The warning I get is that something went wrong (I believe TD030 is the error code). Make sure your are close to it (inches), make sure the device is online with the same wifi as the phone (they are). I don't know what to do. Everything appears to be working fine on the thermostat, if just won't connect to the app.
