Instant water heater does not heat water properly

Despite investing a large amount installing an instant water heater, it does not heat the water properly
Instant water heaters are supposed to switch off automatically, when the cut off temperature is reached.
However due to hacking or other reasons, the water heater is not heating water properly.
Only initially when the water heater is switched on , the user is getting some hot water and later the water is not getting heated at all. The thermostat regulating the heater may be switching off immediately
So it is better to use an electric kettle and get very hot water quickly, since there is no thermostat to switch it on and off.