This time of year, your incoming well water could be very close to freezing. Tankless systems are sort of like waving your hand through a candle flame. The faster you do it, the less heat your hand gets. It really doesn't matter what you've set the outlet temp to, at typical flows with frigid incoming water, it can only add so much heat as the water passes by the heat plates. 120A at 240vac is just under 100KBTU, which is quite small for a tankless system as a more typical sized unit in say gas is closer to 200K. Assuming it actually turns on, at say 2.5gpm, it should be able to raise the water temp around 80-degrees. That's a theoretical max, but it isn't a perfect system, so it's probably closer to 70-degrees. Now, if the incoming water is at say 35, add 70, then lose some in the distance from the heater to your shower, and it might reach body temp IF your valve will let you use all hot (many won't!). Smaller flow, potentially higher delta Temp, but there's a limit on how hot it will let it get.
Electric tankless systems are rarely a good choice for a whole home water heater.