Diagnosis
Yeah, this one's common. It's usually a conflict with your router's band steering or a weak Wi-Fi signal at the thermostat's location (often on an interior wall). The ecobee Lite only supports 2.4GHz networks.
How to Fix It
Log into your router's admin settings. Temporarily disable any 'Smart Connect', 'Band Steering', or 'Wi-Fi Agile Multiband' features. Create a separate, dedicated 2.4GHz network with a simple name (no special characters). Forget the network on the ecobee and reconnect it to this new, pure 2.4GHz SSID.
If That Didn't Work
The signal might just be too weak. Use a Wi-Fi analyzer app on your phone to check signal strength at the thermostat. If it's poor, consider a Wi-Fi extender that creates a strong 2.4GHz network, or reposition your router if possible.
Heads Up
The ecobee doesn't play nice with WPA3-only security. Make sure your 2.4GHz network is using WPA2/WPA3 mixed mode or just WPA2.