Smart Thermostat broken and no hot water

Hi. I have a smart thermostat and several tado trvs. All have been working well. I mainly use the smart thermostat to control the hot water but it is also in a bathroom that does not have a tado trv to keep the bathroom warm when all other rooms are off.

The smart thermostat has stopped working and we have had no hot water. I think I have temporarily fixed this by having it so the receiver light is permanently on (am waiting to see but water is now Luke warm!).

Obviously I want to permanently sort this problem. Initially I thought I would buy a new wireless smart thermostat but you don’t seem to be able to buy one on its own without a whole starter kit.

So I suppose my questions are:

Can I buy a smart wireless thermostat on its own? If not then do I need one to control the hot water or is there another way?

Could I buy a wireless temperature sensor to control the hot water (is it the same as a smart thermostat).

Thanks for the help!