Schematic

It's pretty simple, isn't it?

The RESETN output remains low until the power voltage (V1) reaches 2.7V.

The voltage reference is the base-emitter junction, (partially) temperature compensated by the D1 diode. See simulation below.

LTSpice Simulation

Blue line: power voltage. It's a ramp up to 3.3 Volts.

Yellow lines: RESETN output at various temperatures (Celsius degrees)

As it is evident, the temperature compensation is far from perfect but it's enough to let the RESETN to stay low, resetting the ESP32, if the power voltage is below 2.7 Volts

Hardware

Please not that R4 is changed with a variable resistor to tune the RESETN to stay low up to 2.7 Volts.  

Settings

• Set the power voltage to 2.7 Volts
• Set the external temperature at the lower level you mean to operate (your fridge should be around 4-6 degrees centigrade)
• Turn the R4 resistive trimmer until the RESETN is stable at 0 - 0.1 Volts
• Now setting the voltage to 2.8 - 2.9 Volts the RESETN should go up to 2.8 2.9 Volts.

You're done!