OK, like lots of folks I have been having the problem of setting up multiple thermostats, and having the extra therms fail to be added to the Nest (or Google Home apps). I had this happen back to back and figured some stuff out. This only applies if, when you are adding a thermostat device, you get, either:
1) an error where the second thermostat can't communicate with the first one and fails (TD030 errror)
2) when you add the second thermostat, after trying to communicate to the thermostat, the Nest app "can't reach the Nest servers, try again?" error and then dumps you back to the main screen.
If you are having one of these problems, follow these steps:
1) If you were ever had a NEST login (vs a Google login), then your old Nest account might still be the "owner of the device". To fix this, log out of the Nest app, and then login with your Nest account (the option is on the bottom of the login screen). You might have to use the forgot your password option. Get in to the app with the old login, and then for any thermostats, click on them, then click on the gear and choose "Remove thermostat". I think this is a common error, because Google has nagged everyone so long to upgrade their Nest account, but that process is buggy and often leaves the thermostats in a weird limbo where they are owned by the Nest account. Login to the old account and remove them. This was my problem and was terrible to figure out.
2) Once that is done, go to the thermostat that doesn't work. Goto settings, and the reset, and then reset "the WNN settings" or "the network settings". The thermostat will then reboot.
3) After restarting, setup the wifi network again.
4) Now, go to the Nest app connection on the thermostat, and use get entry code. It should now go request a code from Google. Do this twice in a row, and make sure you get a different entry code each time. If you get the same one, then it is still using the cached network settings, go back to step 3 and reset the network/WNN settings again.
5) Now, add a thermostat in the Nest app, select the "continue with scanning" option, say that it is "already installed" and then choose the entry code that the thermostat is displaying.
If you do all those steps, it should work perfectly.
If you have wifi connectivity problems during the install, you can always set them all up on your phone hotspot and then after they are all added to the app, you can flip them back to your wifi system.
