No lights on thermostat

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Hi,
Hope someone can help.
5/6 thermostats are working. For 2 months since installation, all 6 thermostats worked. But today the one in the kitchen has stoped working. No lights on when I turn the adjuster. Zero lights.
The app connects but nothing happens when I change the temp. Radiator stays cold.
I tried changing batteries and I tried it on another radiator. Seems dead.
Any one have a similar prob?

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