I've never used a Steibel Eltron tank heater (but have owned their tankless), but neither the AOSmith nor Rheem require an app to use. Both can be placed into "Electric Only" mode via front control buttons == no noise.

The Rheem specifically has a time-of-use relay/powerbox, so you can put a timer onto it which will interrupt ALL heating with a rotary dial/timer. This is entirely mechanical, without computer/app.

Sounds like your unit is cooked (36 hours continues heatpump is very bad for the pump).