Getting a 403 forbidden error while uploading a document using sharepoint 2013 REST API

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Can someone please shed some light on this? I am working on a task to generate an on demand spreadsheet/document and upload the generated document to SharePoint 2013 (on-prem) from .net core 2.0 application. I am using Epplus.core libraries to generate the spreadsheet. Using CSOM is not an option for me as I didn't find any nuget libraries which support .net core 2.0 framework so, I am trying to use SP REST API to upload the generated document.

Here's my sample code to test upload a document to sharepoint.

<pre>using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string filePath = @"C:\test.xlsx"; //File to be uploaded
            string siteurl = "https://SpSite"; //site on which file needs to be uploaded (don't put  / at end)
            string documentlibrary = "DocumentLibrary"; //Document library where file needs to be uploaded
            bool filestatus = UploadUsingRest(siteurl, documentlibrary, filePath);
            if (filestatus)
            {
                Console.WriteLine("File uploaded successfully");
            }
            else
            {
                Console.WriteLine("Error uploading file");
            }
        }

        //Method to upload File
        public static bool UploadUsingRest(string siteurl, string libraryName, string filePath)
        {
            bool status = false;
            byte[] binary = System.IO.File.ReadAllBytes(filePath);
            string fname = System.IO.Path.GetFileName(filePath);
            string result = string.Empty;
            //Url to upload file
            string resourceUrl = siteurl + "/_api/web/GetFolderByServerRelativeUrl('" + libraryName + "')/Files/add(url='" + fname + "',overwrite=true)";
            HttpWebRequest wreq = HttpWebRequest.Create(resourceUrl) as HttpWebRequest;
            wreq.UseDefaultCredentials = false;
            //credential who has edit access on document library
            NetworkCredential credentials = new System.Net.NetworkCredential("username", "password", "domain");
            wreq.Credentials = credentials;
            //wreq.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f");
            //Get formdigest value from site
            string formDigest = GetFormDigestValue(siteurl, credentials);            
            wreq.Headers.Add("X-RequestDigest", formDigest);
            wreq.Method = "POST";
            wreq.Timeout = 1000000; //timeout should be large in order to upload file which are of large size
            wreq.Accept = "application/json; odata=verbose";
            wreq.ContentLength = binary.Length;
            try
            {
                using (System.IO.Stream requestStream = wreq.GetRequestStream())
                {
                    requestStream.Write(binary, 0, binary.Length);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }

            try
            {
                WebResponse wresp = wreq.GetResponse();
                using (System.IO.StreamReader sr = new System.IO.StreamReader(wresp.GetResponseStream()))
                {
                    result = sr.ReadToEnd();
                    status = true;
                    return status;
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return status;
                throw;
            }
        }

        //Method which return form digest value
        private static string GetFormDigestValue(string siteurl, NetworkCredential credentials)
        {
            string newFormDigest = "";
            HttpWebRequest endpointRequest = (HttpWebRequest)HttpWebRequest.Create(siteurl + "/_api/contextinfo");
            endpointRequest.Method = "POST";
            endpointRequest.ContentLength = 0;
            endpointRequest.Credentials = credentials;
            endpointRequest.Accept = "application/json;odata=verbose";
            //endpointRequest.UserAgent = @"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)";

            try
            {
                HttpWebResponse endpointResponse = (HttpWebResponse)endpointRequest.GetResponse();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }

            try
            {
                WebResponse webResp = endpointRequest.GetResponse();
                Stream webStream = webResp.GetResponseStream();
                StreamReader responseReader = new StreamReader(webStream);
                string response = responseReader.ReadToEnd();
                var j = JObject.Parse(response);
                var jObj = (JObject)JsonConvert.DeserializeObject(response);
                foreach (var item in jObj["d"].Children())
                {
                    newFormDigest = item.First()["FormDigestValue"].ToString();
                }
                responseReader.Close();

            }
            catch (Exception ex)
            {

                Console.WriteLine(ex.Message);
            }

            return newFormDigest;
        }
    }
}

What I have tried:

I have tried using defaultcredentials, tried passing in useragent to the request. Tried adding in an header for FBA as well. It seems like this is an issue with authentication/authorization. Let me know if anything has to be changed on or if adfs has to be disabled etc.