The neoStat is not designed to control a hot water tank directly, it should only used as a Time clock that feeds a supply to a separate mechanical thermostat on the cylinder.
It is the thermostat that then turns the valve and hot water boiler off when the tank is up to temperature.
For electric immersion heaters, we recommend you use a contactor or solid state relay to do the switching
In this configuration the neoStat switches power to the contactor which then feeds the immersion heater
Please note you must use an immersion heater with a built in thermostat.