This is just a conversion from a two wire thermostat to a smart thermostat with no other wires available like the C- Common wire.

Solution? Install an external 24vac transformer and connect one wire to Common connection. The other wire depends on the configuration:

For furnace heating only – power to RC and C

For AC only – power to RH and W1

This is only to power up the smart thermostat when there’s just two wire available to use from the old thermostat. And this will make the wi-fi running on the device for you to control.

But you have to remember that this is only for just two wire connections. And you cannot control the fan as they are probably hard wired at the furnace/AC unit. Not both.

So, if you need full control, install and layout the 5 or 6 wire cable.

Good luck!