You might have crossover problem caused by bad faucet cartridge.http://waterheatertimer.org/Crossover.html
The green standby light should come on when 240Volt power is applied. Page 6 manual.
Both red lights should come on when tankless is activated. Page 6 manual
RTE 13 requires 60 amp breaker and 6 gauge wirehttp://waterheatertimer.org/How-to-wire-Tankless-electric-water-heater.htmlhttp://waterheatertimer.org/pdf/Rheem-TanklessElectricUC_AP15447.pdf
Factory temperature setting is usually default 120 degrees.
Adjust temps page 11 manual
Thermostat calibration is always approximate.
120 degrees is not hot enough to burn the skin.
Exceptionally cold incoming water temperature can cause tankless not to heat enough hot water: install tempering tank to preheat water to room temperature.
Tankless cannot be exposed to more than 11 grains hardness in water. Delime tankless yearly, clean water filter monthly, do not install low-flow restrictions or conserve water since the flow rate must remain high for tankless to activate.
Rheem does not post service manuals for their tankless water heaters.
Call Rheem 800 432-8373
If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7