Update table row in database c# console app

trying to update a column of null values with instagramIds, this is my current approach but the console app just keeps running and doesn't update any values in the database.

public static async Task<InstagramUser> ScrapeInstagram(string url)
        {
            using (var client = new HttpClient())
            {
                var response = await client.GetAsync(url);
                if (response.IsSuccessStatusCode)
                {
                    // create html document
                    var htmlBody = await response.Content.ReadAsStringAsync();
                    var htmlDocument = new HtmlDocument();
                    htmlDocument.LoadHtml(htmlBody);

                    // select script tags
                    var scripts = htmlDocument.DocumentNode.SelectNodes("/html/body/script");

                    // preprocess result
                    var uselessString = "window._sharedData = ";
                    var scriptInnerText = scripts[0].InnerText
                        .Substring(uselessString.Length)
                        .Replace(";", "");

                    // serialize objects and fetch the user data
                    dynamic jsonStuff = JObject.Parse(scriptInnerText);
                    dynamic userProfile = jsonStuff["entry_data"]["ProfilePage"][0]["graphql"]["user"];

                    List<String> columnData = new List<String>();


                    //Update database query 
                    string connectionString = @"Server=myProject-dev-db.cothtpanmcn7.ap-southeast-2.rds.amazonaws.com;Database=Projectdb;User Id=testadmin;Password=U8gs7vb7C7yvakXf;MultipleActiveResultSets=true;Trusted_Connection=False;";

                    using (SqlConnection con = new SqlConnection(connectionString))
                    {
                        //get null values from database  
                        string query = "Select * from ApplicationUser where InstagramId is null";
                        using (SqlCommand command = new SqlCommand(query, con))
                        {
                            command.Connection.Open();

                            using (SqlDataReader reader = command.ExecuteReader())
                            {
                                while (reader.Read())
                                {
                                    columnData.Add(reader.GetString(0));
                                }
                            }
                        }

                        for (int index = 0; index < columnData.Count(); index++)
                        {
                            //get username and scrape info 
                            var instagramInfo = new InstagramUser
                            {   
                                Id = userProfile.id,   
                            };
                            columnData.Add(instagramInfo.ToString());
                        }

                        SqlCommand cmd = new SqlCommand("Update ApplicationUser Set InstagramId = '" + columnData + "'" + "where InstagramUsername =  '" + userprofile.username + "'", con);
                        cmd.Connection.Open();
                        cmd.ExecuteNonQuery();

                    }

                    // create an InstagramUser
                    var instagramUser = new InstagramUser
                    {
                        FullName = userProfile.full_name,
                        FollowerCount = userProfile.edge_followed_by.count,
                        FollowingCount = userProfile.edge_follow.count,
                        Id = userProfile.id,
                        url = url
                    };
                    return instagramUser;
                }
                else
                {
                    throw new Exception($"Something wrong happened {response.StatusCode} - {response.ReasonPhrase} - {response.RequestMessage}");
                }
            }
        }

The current output:

{"FullName":null,"FollowerCount":0,"FollowingCount":0,"Id":"6978701146","url":null}
{"FullName":null,"FollowerCount":0,"FollowingCount":0,"Id":"6978701146","url":null}
{"FullName":null,"FollowerCount":0,"FollowingCount":0,"Id":"6978701146","url":null}
{"FullName":null,"FollowerCount":0,"FollowingCount":0,"Id":"6978701146","url":null}

{"FullName":null,"FollowerCount":0,"FollowingCount":0,"Id":"6978701146","url":null}

My current approach is to create a list, add all instagramIDs which are null to that list. From there I add all instagramIds to that list after scraping Instagram for their usernames e.g. https://www.instagram.com/therock/?__a=1

then I update the column InstagramUsername with their instagram Ids