Insert query cannot be executed using the singleton database connection [duplicate]

This question already has an answer here:

• mysqli_fetch_assoc() expects parameter / Call to a member function bind_param() errors. How to get the actual mysql error and fix it? 1 answer

I am trying to perform an insert query using the DB connection (singleton pattern used) but I keep getting HTTP error 500 But if I try to perform the insert query using a normal DB connection, like creating a new connection every time, rather than using the same connection everywhere, it does show results

Meanwhile, I tried to perform Select query which works perfectly fine

<?php 

class DBtest
{
    private $_mysqliConnectionString; 

    public static $instance;

    private $servername; 
    private $username; 
    private $password; 
    private $dbase;

    public static function getInstance()  //to get only one instance of the DB connection, singleton pattern applied
    {
        if(!isset(self::$instance))
        {
            self::$instance=new DBtest;
        }
        return self::$instance;
    }

    private function __construct()
    {
        $this->servername = "";
        $this->username = ""; 
        $this->password = "";
        $this->dbase = ""; 

        $this->_mysqliConnectionString = new mysqli($this->servername, $this->username, $this->password,$this->dbase);

        if($this->_mysqliConnectionString->connect_error)
        {
            die($this->_mysqliConnectionString->connect_error);
        }

        mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
        $this->_mysqliConnectionString->set_charset("utf8mb4");
    } 

    private function __destruct()
    {
      if ($this->_mysqliConnectionString) $this->_mysqliConnectionString->close();

    }

    private function __clone() {}

    public function ReturnConnectionString()   //returns connection 
    {
        return $this->_mysqliConnectionString;
    }

    public function query($sql)
    {
        $conn= $this->ReturnConnectionString()  ;
        if (mysqli_query($conn,$sql)) 
        {
        echo "<br>Changes have been successfully implemented in the DB";
        } 
        else 
        {
        echo "<br>Error: " . $sql . "<br>" . $conn->error;
        }     
    }

}

$DBobject = DBtest::getInstance();
$sql ="INSERT INTO Users (id,name, email, password)    VALUES (110,'abc','Smth@live.kr','BTS')";

$DBobject->query($sql);
?>