unable to update a varchar (400) column in remote server

It doesn't work is not that helpful, but the lack of "it throws an exception with this message" implies that the update has succeeded as far as SQL is concerned. The most likely culprit is indeed that the remote database does not have any rows which exactly match your WHERE condition - even if your local one does.
The other possibility is that you are causing the problem yourself by concatenating strings to produce your SQL command, and causing an SQL Injection from the data you are trying to insert.
You may not know what SQL Injection is, but you damn well should - given that it can damage or destroy your database without any difficulty. Think about it: what happens if the data you retrieve from the local server contains a quote character? That terminates the highlights string as far as SQL is concerned, and the remainder of the data in the string is interpreted and executed as a command. Use parametrized queries at all times - never, ever concatenate strings to form SQL commands!

using (SqlConnection con = new SqlConnection(strConnect))
    {
    con.Open();
    using (SqlCommand com = new SqlCommand("INSERT INTO myTable (myColumn1, myColumn2) VALUES (@C1, @C2)", con))
        {
        com.Parameters.AddWithValue("@C1", myValueForColumn1);
        com.Parameters.AddWithValue("@C2", myValueForColumn2);
        com.ExecuteNonQuery();
        }
    }