How to delay outside loop

I am using a function outside loop(), which is called after an interrupt. Inside this function, I am setting several relays that need to change after a second or two, depending on my setting. I tried the delay, it doesn't work. millis() isn't working as well; I found out it stops counting when I exit the loop function.

My code looks something like this:

const long on_interval = 2000;
unsigned long currentMillis = millis();
bool hv_bool = false;

void hv_switch() {
    if (digitalRead(hv_pin)==HIGH){
        hv_startup();
    }
}

void hv_startup() {
    unsigned long previousMillis = 0;
    digitalWrite(pre_charge, HIGH);
    digitalWrite(main_charge, LOW);
    //Wait interval
    previousMillis = currentMillis;
    while (currentMillis - previousMillis < on_interval) {
        Serial.println(millis());
        //Wait
    }
    digitalWrite(pre_charge, LOW);
    digitalWrite(main_charge, HIGH);
    hv_bool = true;
    loop();
}

void setup(){
    //Set some ports

    //Initiate interrupt 
    attachInterrupt(digitalPinToInterrupt(hv_pin), hv_switch, RISING);
}

void loop() {
    //Low voltage loop after startup, high voltage off
    while (hv_bool == false) {
        Serial.println("Waiting on high voltage");
        digitalWrite(hv_led, LOW);
        digitalWrite(pre_charge, LOW);
        digitalWrite(main_charge, LOW);
    }
}