Sometimes WiFi thermostats are placed on the edge of the signal range, this can cause intermittent connectivity. Alternatively, obstructions of the thermostat signal is also a factor, anything that could potentially block the signal such as metal cabinets, large mirrors, or very thick walls could have an impact on how well the signal gets through. The easiest way to fix the issue may be to add another wireless access point and put it in between the router and the thermostat to boost the signal.