Try these steps:
- Reset the devices to factory defaults
- Reboot the ST hub
- Bring the device within 5ft of the hub if possible and then exclude it again
- Now try to pair the device
This topic may also help with more details guides: How to migrate from Wink to SmartThings
NOTE: Your 2Gig thermostat may not be possible to reset by pressing the black button, that’s a soft reset. See this topic first post on how to do a factory reset. These are basically rebranded CT-xxx thermostats.