Smart Thermostat disconnects multiple times a day - works when I pull batteries

Wierd problem on one of my Tado systems:

The bridge is fine and stays connected all the time.

Several times a day the Smart thermostat becomes disconnected from the system.

Sometimes it reconnects after a few minutes.

Rebooting the bridge does nothing, but I

I can force it to reconnect by removing the smart therm's batteries and reinserting them.

Brisge is working fine, the batteries are new -

can't work out what's wrong? - has anyone experienced similar?