Ok i intall tankless water heaters and sell them and repair them . The trouble your are hving is more than likely ,the mixing valve on your shower has a ajustment on it to prevent you from burning your self . that is the number one reason in showers when water never get to temp . What it is doing is addng cold water to your shower for you bummer . Take handle off and adjust water temp to your hottest setting . Some times you have to remove the low flow washer in shower head to get the right flow . also every time you start up your hot water it shouldbe set all the way ot other wise withe the battery 2 D models the unit will not sence flow and shut off the unit ,with some units it is a fine balancing act to get them adjusted right. If your buying a tankless water heater always buy a blower type unit that has a computer that runs it . You will be happy . If the Battery operated unit is for a cottage or a smallcabin then your just going to have to fix the pumbing to make it work right many of the safety features of the newer showers are not made to work well with tankless water heaters . 
Bosch posted a manual about overcoming fluctuating temperatures.
It might help or not.
http://waterheatertimer.org/pdf/Bosch_Overcoming_fluctuating_temps.pdf
Other Marey information:
1-877-727-0277
http://www.marey.com/panelwi/articulos/pdf/7b2d95_2011_02_23_manual_gas_english_baja.pdf
http://waterheatertimer.org/pdf/Marey_daily-maintenance.pdf
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