When a thermostat was paired before, and now fails to re-pair repeatedly, it means there is "wifi junk" between your device, Mysa and the router that's not being cleared. To remove the WiFi Junk, please do the following:
1) Delete the problematic thermostat from the Mysa app
Mysa app > tap thermostat > tap settings in the right-hand corner > tap the trash can icon
2) Delete the problematic thermostat from the Apple Home App.
Apple Home app > tap edit > tap the thermostat you wish to remove > scroll down and tap Remove Accessory
3) Reboot the iOS device that you are using to connect Mysa with.
4) Put the thermostat into pairing mode by
Factory reset your Mysa thermostat (hold down on the up and downs arrows simultaneously for 10-15 seconds). The thermostat should reboot and the smiley face appears.
5) Reboot the thermostat by turning the breaker off and then back on
6) Attempt to pair the thermostat again.
If pairing fails again, repeat steps 3-5 multiple times.