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В коллбэке заполняйте массив, объявленный глобально.
Так будет проще всего.

Спасибо! Заполнил, работает!

#include <BLEDevice.h>
#include <BLEUtils.h>
#include <BLEScan.h>
#include <BLEAdvertisedDevice.h>
//#include <BLEBeacon.h>

#define ENDIAN_CHANGE_U16(x) ((((x)&0xFF00) >> 8) + (((x)&0xFF) << 8))
String device_data[4] ;
String address;
String rssi;
String uuid;
String power;
class IBeaconAdvertised: public BLEAdvertisedDeviceCallbacks {
  public:
    // Обратный вызов при обнаружении BLE
    void onResult(BLEAdvertisedDevice device) {
      if (!isIBeacon(device)) {
        return;
      }
      printIBeacon(device);
    }

  private:
    // Определение пакетов iBeacon
    bool isIBeacon(BLEAdvertisedDevice device) {
      if (device.getManufacturerData().length() < 25) {
        return false;
      }
      if (getCompanyId(device) != 0x0100) { // для Apple 0x4C00 MSB
        return false;
      }
      if (getIBeaconHeader(device) != 0x1502) {
        return false;
      }
      return true;
    }

    // Получить идентификатор компании
    unsigned short getCompanyId(BLEAdvertisedDevice device) {
      const unsigned short* pCompanyId = (const unsigned short*)&device
                                         .getManufacturerData()
                                         .c_str()[0];
      return *pCompanyId;
    }

    // Получить заголовок iBeacon
    unsigned short getIBeaconHeader(BLEAdvertisedDevice device) {
      const unsigned short* pHeader = (const unsigned short*)&device
                                      .getManufacturerData()
                                      .c_str()[2];
      return *pHeader;
    }

    // Получить UUID
    String getUuid(BLEAdvertisedDevice device) {
      const char* pUuid = &device.getManufacturerData().c_str()[4];
      char uuid[64] = {0};
      sprintf(
        uuid,
        "%02X%02X%02X%02X-%02X%02X-%02X%02X-%02X%02X-%02X%02X%02X%02X%02X%02X",
        pUuid[0], pUuid[1], pUuid[2], pUuid[3], pUuid[4], pUuid[5], pUuid[6], pUuid[7],
        pUuid[8], pUuid[9], pUuid[10], pUuid[11], pUuid[12], pUuid[13], pUuid[14], pUuid[15]
      );
      return String(uuid);
    }

    // Получить txPower
    signed char getTxPower(BLEAdvertisedDevice device) {
      const signed char* pTxPower = (const signed char*)&device
                                    .getManufacturerData()
                                    .c_str()[24];
      return *pTxPower;
    }

    // Последовательный вывод информации iBeacon
    void printIBeacon(BLEAdvertisedDevice device) {
   /*  Serial.printf("addr:%s rssi:%d uuid:%s power:%d\r\n",
                    device.getAddress().toString().c_str(),
                    device.getRSSI(),
                    getUuid(device).c_str(),
                    *(signed char*)&device.getManufacturerData().c_str()[24]);*/
    
device_data[0] = device.getAddress().toString().c_str();
device_data[1] = device.getRSSI();
device_data[2] = getUuid(device).c_str();
device_data[3] = *(signed char*)&device.getManufacturerData().c_str()[24];
}
};

void setup() {
  Serial.begin(115200);
  BLEDevice::init("");
}

void loop() {
  //Serial.println("start.");
  BLEScan* scan = BLEDevice::getScan();
  scan->setAdvertisedDeviceCallbacks(new IBeaconAdvertised(), true);
  scan->setActiveScan(true);
  scan->start(60);
  address = device_data[0];
  rssi = device_data[1];
  uuid = device_data[2];
  power = device_data[3];
   Serial.print("address:  ");
  Serial.println(address);
   Serial.print("rssi:  ");
   Serial.println(rssi);
    Serial.print("uuid:  ");
  Serial.println(uuid);
   Serial.print("power: ");
  Serial.println(power);
 
}