Steps
If the Netatmo app displays "Thermo Hub unreachable", the Thermo Hub may not be correctly plugged into the power supply, or the internet connection may be unstable or down.
In most cases, you can restore the connection by following a few simple steps:
Unplug your Wi-Fi router from the power supply.
Unplug the Thermo Hub from the power supply.
Reconnect the Wi-Fi router and wait until the internet connection is restored.
Plug the Thermo Hub into the power supply. The LED should blink in green.
Comments
If you have changed your router's Wi-Fi password or installed a Wi-Fi repeater, you need to reconfigure the network access of the Thermo Hub by clicking on the "Configure Wi-Fi" button in the error message of the Netatmo app.
⚠️There is no need to uninstall the Thermo Hub from the app. All personal data, including historic curves, settings, and weekly schedules, would be lost in this operation!
The Thermostat will continue to work even if the Thermo Hub is unavailable. (More info)
If you temporarily lose your internet connection, the Netatmo devices will automatically reconnect once it is available again.
Updated