Help with Sain Smart 24V Relay Cycle Timer Module
The cycle timer you linked to seems to require 24 VDC. The thermostat will be fed with 24 VAC. Do not use that cycle timer!

You can wire a cycle timer just like a thermostat, in parallel to the existing thermostat. You can wire it near your furnace if that's more convenient; it will just connect the two conductors to call for heat at the set intervals, whether the thermostat is doing the same thing or not.

Look for one that's designed for HVAC use, e.g. https://www.supplyhouse.com/ICM-Controls-ICM305-ICM305-Duty-Cycle-Timer-Time-Delay-in-Minutes