Connect websocket on esp8266 to local server using IP

I started working a small "smart home" project to learn about embedded development and Rust. I have

• an ESP8266 with a DHT to measure humidity and temperature, written in C++
• a server written in Rust that has a route with a very small FE (see next point) and routes for websockets to send and receive data
• a small FE which is currently a few lines of HTML and JS that connects to above mentioned server and reads the data sent by the sensor

The "production" version of the server is hosted and has a domain name which I can use to connect to it. The microcontroller and FE can connect using wss and https respectively using the domain name.

I also want a setup that I can use locally to debug. Meaning, the server is running on 127.0.0.1 and the microcontroller can connect to it. In the end I want to have a build flag that I can set to define what is being built. Ideally the local setup would also have SSL encryption to better approximate the "production" setup. I already have a locally created certificate that I could use but as a first step a simple connection is enough.

My problem is that I can't get the microcontroller to connect to the local server running on 127.0.0.1

I'm using PlatformIO with the following libraries:

• adafruit/DHT sensor library@^1.4.6
• adafruit/Adafruit Unified Sensor@^1.1.14
• gilmaimon/ArduinoWebsockets@^0.5.3

Here is the code of the microcontroller:

#include <Arduino.h>
#include <DHT.h>
#include <DHT_U.h>
#include <ESP8266WiFi.h>
#include <ArduinoWebsockets.h>

#define DHTTYPE    DHT11
#define DHTPIN 2

DHT_Unified dht(DHTPIN, DHTTYPE);

const char* const public_address = "wss://mydomain.com/sensor/";
const char public_fingerprint[] PROGMEM = "<fingerprint>";

const char* const local_address = "wss://127.0.0.1:9231/sensor/";
const char local_fingerprint[] PROGMEM = "<fingerprint>";

const bool IS_LOCAL_SETUP = true; // for now simple bool switch

bool connected = false;

using namespace websockets;
WebsocketsClient client;

void onMessageCallback(WebsocketsMessage message) {
  // ...
}

void onEventsCallback(WebsocketsEvent event, String data) {
  // ...
}

void setup() {
  Serial.begin(115200);

  while (!Serial) {
    ; // Wait for serial
  }

  WiFi.mode(WIFI_STA);
  int count = WiFi.scanNetworks(false, true, 0, NULL);
  Serial.printf("Number of wifi networks: %d\n", count);

  String ssid = "<ssid>";

  Serial.println(ssid);

  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, "<password>");

  while (WiFi.status() != WL_CONNECTED && WiFi.status() != WL_CONNECT_FAILED)
  {
    delay(750);
    Serial.print('*');
  }

  Serial.println();

  if (WiFi.status() == WL_CONNECT_FAILED)
  {
    Serial.println("Error when connecting to wifi");
    return;
  }

  Serial.println("WiFi connection successful");
  Serial.println(WiFi.localIP());

  // run callback when messages are received
  client.onMessage(onMessageCallback);
  
  // run callback when events are occuring
  client.onEvent(onEventsCallback);

  if (IS_LOCAL_SETUP) {
    client.setFingerprint(local_fingerprint);
  } else {
    client.setFingerprint(public_fingerprint);
  }

  bool res;
  // Connect to server
  if (IS_LOCAL_SETUP) {
    res = client.connect(local_address);
  } else {
    res = client.connect(public_address);
  }
  if (!res) {
    Serial.println("Failed to connect");
    return;
  }

  connected = true;

  dht.begin();
  // printing sensor info
}

void loop() {
  delay(2000);

  if (!connected) {
    return;
  }

  client.poll();

  // Get temperature event and print its value.
  // ...

  // Send a message
  std::string msg = "...";
  bool res = client.send(msg.c_str());
  if (!res) {
    Serial.println("Failed to send message");
    return;  
  }
}

I set the defines DEBUG_ESP_SSL and DEBUG_ESP_PORT to get more logs from the WSP8266WiFi library but can't see anything useful:

SDK:2.2.2-dev(38a443e)/Core:3.1.2=30102000/lwIP:STABLE-2_1_3_RELEASE/glue:1.2-65-g06164fb/BearSSL:b024386
fpm close 1
mode : sta(c4:5b:be:55:92:4d)
add if0
scandone

**scandone
state: 0 -> 2 (b0)
*state: 2 -> 3 (0)
state: 3 -> 5 (10)
add 0
aid 17
cnt

connected with <SSID>, channel 6
dhcp client start...
ip:<ip>,mask:255.255.255.0,gw:<>

Setting fingerprint
Creating client
BSSL:connect: Unable to connect TCP socket

pm open,type:2 0

No matter what, the client.connect call returns 0. From the logs I can see that the connection fails when calling WiFiClient::connect, though I don't know why.

What I have tried:

• Use localhost instead of the IP in a desperate try as I thought maybe its not possible to pass an IP directly - no difference
• Convert wss to ws and remove the fingerprint - no difference

Is there something obvious I'm missing? I read that firewall might be an issue but since I can access the FE and the FE can create a websocket connection to the local server I doubt that is the issue.

Please let me know if you need the code for the server.

Thanks a lot for any help and tips