Search Amazon or ebay for Zigbee Centralite Xfinity 3156105.
If it's a 24VAC control circuit, this is one should work. Battery powered, 2-wire compatible, adjustable hysteresis. It's not the prettiest option, but they work well and the price is right.
For reliability reasons, I would recommend using a dedicated thermostat. It will continue to operate as intended even if the hub goes down.
