Thermostat not calling for heat.

So I decided to buy a used 3rd gen thermostat. I got everything installed. I made sure I hooked everything up correctly as well… When the thermostat is set to heat though it seems like the thermostat doesn’t actually call for heat when in Gas and Forced air modes. I did change the heat source mode to electric and it did turn the blower on ( we have gas so still no heat was made) I also spent time on a support chat with nest for almost an hour trying to troubleshoot with them. Going through resets, updates, double and triple checking wiring to no avail. I’m about to tear the thing apart to see if maybe there is just a loose connection somewhere??

If anyone has any ideas, please feel free to let me know!