If a thermostat has been previously been connected to a neoHub and either the neoHub has been replaced or the neoHub has gone through a factory reset, (status light is off). Any thermostat connected to it will need to be factory reset. Once done you should be able to add back to the Heatmiser Neo App using the pairing process.
This is how you would reset our range of neoStats which uses a neoHub for App control.
Hard wired neoStats or neoStat V2
There is a single screw at the bottom of the thermostat
How to Factory Reset a hard wired neoStat or a neoStat V2
NeoStat Pro
How to Factory Reset a NeoStat Pro
NeoStat Touch or NeoStat Touch-e White or Carbon
How to Factory Reset a NeoStat Touch
NeoAir, NeoAir V2, NeoAir V2m NeoAir V3
Battery Tray on the underside of the thermostat. NeoAir V3 has buttons
How to Factory Reset a neoAir and connect to an RF Switch
How to Factory Reset a neoAir when connected to an RF Switch V2
How To Factory Reset a neoAir which is connected to a UH8-RF wiring centre
How to Factory Reset a neoAir which is connected to the UH8-RF V2 wiring centre