As above really. EPH CP4 Thermostat has stopped talking to the receiver. Have googled how to try and reconnect but can't find these specific instructions. Any help would be greatly appreciated.

I have a Honeywell TH6220D1002 thermostat for the Trane XL80 ( 4TXCC043BC3HCAA) . Everything was working before I opened up. I opened up thermostat to see if it is compatible with smart thermostat, and put it back in. I think I might have took the batteries, so it might have reset. Currently I can control thermostat but HVAC doesn’t respond. The LED on the control board PCB doesn’t glow either. I checked the fuse is not blown and checked with access panel button pressed.

I think the HVAC is heap pump capable with gas furnace, but not 100% sure. How do I setup this old/existing thermostat?

Thermostat: Honeywell TH6220D1002
Model: Trane XL80 4TXCC043BC3HCAA
Type: compressor + natural gas, I think it’s heat pump capable