Clear your codes. Charge up the HV battery full. Drive in EV mode for 10 miles or so, then select mountain mode. Will the car switch to mountain mode and run properly while maintaining that 3-4 bars of battery range on the guessometer? Your case sounds somewhat similar to the one several threads below. What is the condition of your 12V battery (it’s in the back under the floor)? Weak 12V batteries can cause issues.