Except for the Smart, Smart SI, EMS, and EMS SI thermostats, this article applies to all ecobee thermostats.
There are a few things to consider if your ecobee won't connect to Wi-Fi or the Internet:
1.) Check that your router is turned on and broadcasting on a 2.4 GHz network. If you have a dual-band router, connect to the 2.4 GHz band rather than the 5GHz channel.
UPDATE: Because SmartThermostat with Voice Control is dual-band compatible, this concern does not apply to it (i.e. supports connections on both the 2.4 GHz and 5 GHz bands).
2.) Check your thermostat's firmware for updates by heading to MAIN MENU > ABOUT > VERSION.
3.) Verify that your router's encryption type is configured to WPA/WPA2-AES. We recently upgraded our security protocols, which removed support for TKIP encryption, therefore your router settings may need to be updated.
4.) On your thermostat, navigate to MAIN MENU > ABOUT > WI-FI to check the signal strength. Connection problems could be caused by a signal strength of less than 75%.