Thermo Watch will work for you if you are on a Nest account created before September 1st 2019, that has not been migrated to a Google account. Right now it does not support login with a Google Nest account created after the September 1st. Support for Google accounts is coming in early 2020. Note that if you are on a Nest account, migration to Google account is not mandatory.
The simplest and quickest way of controlling your Nest Thermostat*
Nest Learning Thermostat,
Together at last.
*Aside from always standing right next to it.