How to connect using CURL?

Question

I'm learning how to use CURL properly, and according to all the examples (the documentation is a pain) my code should work, but for some reason sometimes it connects and other times it won't.

I did check if there was a firewall problem, or the antivirus interfering, but both are turn off and the problem persists.

The main idea is to connect to a local server (rpi), and in the future to an external server for backup/updates.

My code is as follows. Here's the callback function, and the actual function that does all the work, the different URLs are for example purposes.

static std::size_t callback(const char* in,std::size_t size, std::size_t num, std::string* out){
    Silo* silo = new Silo();    
    const std::size_t totalBytes(size * num);
    std::string data = std::to_string(totalBytes);
    silo->Log("Total Bytes recive " + QString::fromStdString(data));
    out->append(in, totalBytes);
    return totalBytes;
}  

void Server::RPI_Request(){
    Silo* silo = new Silo();
    //curl_global_init(CURL_GLOBAL_ALL);
    CURL *curl = curl_easy_init();
    
    const std::string url_A("http://date.jsontest.com/");
    const std::string url_B("https://jsonplaceholder.typicode.com/todos/1");
    const std::string url_C("https://www.google.com/");
    const std::string url_D("https://stackoverflow.com/");
    
    if (curl){
        CURLcode res;
       
        // set Ip Direction
        curl_easy_setopt(curl, CURLOPT_URL, url_C.c_str() );
        // Don't bother trying IPv6, which would increase DNS resolution time.
        curl_easy_setopt(curl, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4);

        // Don't wait forever, time out after 10 seconds.
        silo->Log("antes de timeout");
        curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10);
 
        // Follow HTTP redirects if necessary.
        //curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);

        // Response information.
        long httpCode(0);
        std::unique_ptr<std::string> httpData(new std::string());
        
        // Hook up data handling function.
        silo->Log("antes de write function");
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, callback);
        
        // Hook up data container (will be passed as the last parameter to the
        // callback handling function).  Can be any pointer type, since it will
        // internally be passed as a void pointer.
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, httpData.get());

        // Run our HTTP GET command, capture the HTTP response code, and clean up.
        silo->Log("antes de easy perform");
        res = curl_easy_perform(curl);
        curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &httpCode);
        
        silo->Log("Respuesta de httpCode: " + QString::number(httpCode));
        if (res != CURLE_OK){
          silo->Log("Hay pedo no se conecto " + QString::fromStdString(url_C) );
          
        } else {
          silo->Log("Coneccion establecida con " + QString::fromStdString(url_C));   
          
        }
     
        curl_easy_cleanup(curl);
        //curl_global_cleanup();
    }
}