I was building an IoT device and uses mongodb atlas as my backend. My ESP32 has to POST data to the database. I uses arduino IDE and here’s my code:

#include <Arduino.h>
#include <WiFi.h>
#include <HTTPClient.h>
#include <WiFiClientSecure.h>
#include <ArduinoJson.h>

/************ Useful Setup Constants *****************/


const char* AtlasAPIEndpoint = "https://ap-southeast-1.aws.data.mongodb-api.com/app/data-<id>/endpoint/data/v1/action/insertOne";
const char* AtlasAPIKey = <my_api_key>;
const char* ssid = "*******";
const char* password = "*********";

/* This one Works with Atlas OK*/

const char* rootCACertificate = 
                                "-----BEGIN CERTIFICATE-----n" 
                                "MIIF6TCCA9GgAwIBAgIQBeTcO5Q4qzuFl8umoZhQ4zANBgkqhkiG9w0BAQwFADCBn" 
                                "iDELMAkGA1UEBhMCVVMxEzARBgNVBAgTCk5ldyBKZXJzZXkxFDASBgNVBAcTC0pln" 
                                "cnNleSBDaXR5MR4wHAYDVQQKExVUaGUgVVNFUlRSVVNUIE5ldHdvcmsxLjAsBgNVn" 
                                "BAMTJVVTRVJUcnVzdCBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3JpdHkwHhcNMTQwn" 
                                "OTEyMDAwMDAwWhcNMjQwOTExMjM1OTU5WjBfMQswCQYDVQQGEwJGUjEOMAwGA1UEn" 
                                "CBMFUGFyaXMxDjAMBgNVBAcTBVBhcmlzMQ4wDAYDVQQKEwVHYW5kaTEgMB4GA1UEn" 
                                "AxMXR2FuZGkgU3RhbmRhcmQgU1NMIENBIDIwggEiMA0GCSqGSIb3DQEBAQUAA4IBn" 
                                "DwAwggEKAoIBAQCUBC2meZV0/9UAPPWu2JSxKXzAjwsLibmCg5duNyj1ohrP0pILn" 
                                "m6jTh5RzhBCf3DXLwi2SrCG5yzv8QMHBgyHwv/j2nPqcghDA0I5O5Q1MsJFckLSkn" 
                                "QFEW2uSEEi0FXKEfFxkkUap66uEHG4aNAXLy59SDIzme4OFMH2sio7QQZrDtgpbXn" 
                                "bmq08j+1QvzdirWrui0dOnWbMdw+naxb00ENbLAb9Tr1eeohovj0M1JLJC0epJmxn" 
                                "bUi8uBL+cnB89/sCdfSN3tbawKAyGlLfOGsuRTg/PwSWAP2h9KK71RfWJ3wbWFmVn" 
                                "XooS/ZyrgT5SKEhRhWvzkbKGPym1bgNi7tYFAgMBAAGjggF1MIIBcTAfBgNVHSMEn" 
                                "GDAWgBRTeb9aqitKz1SA4dibwJ3ysgNmyzAdBgNVHQ4EFgQUs5Cn2MmvTs1hPJ98n" 
                                "rV1/Qf1pMOowDgYDVR0PAQH/BAQDAgGGMBIGA1UdEwEB/wQIMAYBAf8CAQAwHQYDn" 
                                "VR0lBBYwFAYIKwYBBQUHAwEGCCsGAQUFBwMCMCIGA1UdIAQbMBkwDQYLKwYBBAGyn" 
                                "MQECAhowCAYGZ4EMAQIBMFAGA1UdHwRJMEcwRaBDoEGGP2h0dHA6Ly9jcmwudXNln" 
                                "cnRydXN0LmNvbS9VU0VSVHJ1c3RSU0FDZXJ0aWZpY2F0aW9uQXV0aG9yaXR5LmNyn" 
                                "bDB2BggrBgEFBQcBAQRqMGgwPwYIKwYBBQUHMAKGM2h0dHA6Ly9jcnQudXNlcnRyn" 
                                "dXN0LmNvbS9VU0VSVHJ1c3RSU0FBZGRUcnVzdENBLmNydDAlBggrBgEFBQcwAYYZn" 
                                "aHR0cDovL29jc3AudXNlcnRydXN0LmNvbTANBgkqhkiG9w0BAQwFAAOCAgEAWGf9n" 
                                "crJq13xhlhl+2UNG0SZ9yFP6ZrBrLafTqlb3OojQO3LJUP33WbKqaPWMcwO7lWUXn" 
                                "zi8c3ZgTopHJ7qFAbjyY1lzzsiI8Le4bpOHeICQW8owRc5E69vrOJAKHypPstLbIn" 
                                "FhfFcvwnQPYT/pOmnVHvPCvYd1ebjGU6NSU2t7WKY28HJ5OxYI2A25bUeo8tqxyIn" 
                                "yW5+1mUfr13KFj8oRtygNeX56eXVlogMT8a3d2dIhCe2H7Bo26y/d7CQuKLJHDJdn" 
                                "ArolQ4FCR7vY4Y8MDEZf7kYzawMUgtN+zY+vkNaOJH1AQrRqahfGlZfh8jjNp+20n" 
                                "J0CT33KpuMZmYzc4ZCIwojvxuch7yPspOqsactIGEk72gtQjbz7Dk+XYtsDe3CMWn" 
                                "1hMwt6CaDixVBgBwAc/qOR2A24j3pSC4W/0xJmmPLQphgzpHphNULB7j7UTKvGofn" 
                                "KA5R2d4On3XNDgOVyvnFqSot/kGkoUeuDcL5OWYzSlvhhChZbH2UF3bkRYKtcCD9n" 
                                "0m9jqNf6oDP6N8v3smWe2lBvP+Sn845dWDKXcCMu5/3EFZucJ48y7RetWIExKREan" 
                                "m9T8bJUox04FB6b9HbwZ4ui3uRGKLXASUoWNjDNKD/yZkuBjcNqllEdjB+dYxzFfn" 
                                "BT02Vf6Dsuimrdfp5gJ0iHRc2jTbkNJtUQoj1iM=n" 
                                "-----END CERTIFICATE-----n";





WiFiClient client;

char lastid[25] = "0";

void setup() {

  Serial.begin(115200);
  WiFi.mode(WIFI_STA);

  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    Serial.print(".");
    delay(500);
  }
  Serial.println();
  Serial.print("ESP32-CAM IP Address: ");
  Serial.println(WiFi.localIP());
  getLightDefinition();
}

void loop() {

}

void getLightDefinition() {
  WiFiClientSecure client;
  client.setCACert(rootCACertificate);

  HTTPClient https;

  if (!https.begin(client, AtlasAPIEndpoint)) {
    Serial.println("[HTTPS] Unable to connect");
    return;
  }

  https.addHeader("Content-Type", "application/json");
  https.addHeader("Access-Control-Request-Headers", "*");
  https.addHeader("api-key", AtlasAPIKey);

  DynamicJsonDocument payload(1024);
  payload["dataSource"] = "test";
  payload["database"] = "testmakentu";
  payload["collection"] = "test";
  payload["document"]["time"] = "08:00";
  payload["document"]["daysofweek"] = "WED";

  String JSONText;
  serializeJson(payload, JSONText);

  int httpCode = https.POST(JSONText);

  if (httpCode > 0) {
    if (httpCode == HTTP_CODE_OK || httpCode == HTTP_CODE_MOVED_PERMANENTLY) {
      String payload = https.getString();
      Serial.println(payload);
      DynamicJsonDocument description(32687);
      DeserializationError error = deserializeJson(description, payload);
      if (error) {
        Serial.print(F("deserializeJson() failed: "));
        Serial.println(error.c_str());
        return;
      }
    }
  } else {
    Serial.printf("[HTTPS] POST ... failed, error: %sn", https.errorToString(httpCode).c_str());
  }

  https.end();
}

When I run the code, serial monitor shows:[HTTPS] POST ... failed, error: connection refused.

Actually I am just deleting some code from here:
https://github.com/mongodb-developer/xmas_2021_tree_camera/blob/main/ESP32-Arduino/mongo_xmastree/mongo_xmastree.ino

I tried to use postman to POST data and it worked, so I sure that the problem was not that I didn’t have access, and also I know my wifi works ok.

So I guess the problem occurs in the rootCACertificate part. Is my assumption correct? If so, how can I fix it or get another rootCACertificate? Thanks!