DIAGNOSIS The thermostat is losing its Wi-Fi association. This is often due to a router setting that's incompatible with its older Wi-Fi chipset, or a weak signal.
FIX The big fix for most people is to adjust your router's settings. Log into your router and make sure the 2.4GHz Wi-Fi network the T9 uses is not set to "Auto" channel width. Manually set it to 20 MHz channel width. Also, avoid using WPA3-only security; use WPA2/WPA3 mixed mode or just WPA2. Finally, give your thermostat a static IP address or a DHCP reservation in your router settings.
IF THAT DIDN'T WORK If it's still dropping, the signal might be marginal. The T9's Wi-Fi antenna isn't great, especially if your router is far away or through several walls. Consider a Wi-Fi extender that creates a separate 2.4GHz network, or just live with it offline—it'll still control your HVAC fine.
HEADS UP The T9 doesn't support 5GHz Wi-Fi networks at all. If you only broadcast a combined SSID (single name for both bands), some routers aggressively kick older 2.4GHz-only devices off. Creating a separate 2.4GHz-only network for smart home gadgets often solves this.