Php insert data to table NOT working but doesn't show any errors

See more:

PHP

MySQL

Sorry my english is not very good but i need your helps to find what's wrong in below php file.
I'm just learning PHP and am trying to capture info from a form and insert it into a table in a mySQL database.Before doing that,i want to check for duplicate username or email, so i added an if statement but no data was inserted into table and no error was shown.
. Could someone please tell me what I am doing wrong?

Before adding if statement, it worked.

<?php
include('config.php');

    $name = $_POST['name'];
    $user = $_POST['id'];
    $password = $_POST['password'];
    $mail = $_POST['email'];
    $phone = $_POST['phone'];
    $sex = $_POST['sex'];
    $nationality = $_POST['nationality'];
    $dob=$_POST['year'] . '-' . $_POST['month'] . '-' . $_POST['days'];
    $question = $_POST['mistery_question'];
    $reply = $_POST['reply'];

    date_default_timezone_set("Asia/Calcutta");
    $insertdate = date("Y-m-d H:i:s");
    $sql = "INSERT INTO regist (Name, Username, Password, Mail, PhoneNumber, Sex, Religion, DateOfBirth, Question, Answer,date_time) 
            VALUES ('$name', '$user','$password','$email', '$phone', '$sex', '$nationality', '$dob', '$question', '$reply', '$insertdate')";

            mysqli_close($conn);

?>

What I have tried:

After adding an if statement. It does not work

<?php
include('config.php');

    $name = $_POST['name'];
    $user = $_POST['id'];
    $password = $_POST['password'];
    $mail = $_POST['email'];
    $phone = $_POST['phone'];
    $sex = $_POST['sex'];
    $nationality = $_POST['nationality'];
    $dob=$_POST['year'] . '-' . $_POST['month'] . '-' . $_POST['days'];
    $question = $_POST['mistery_question'];
    $reply = $_POST['reply'];

    date_default_timezone_set("Asia/Calcutta");
    $insertdate = date("Y-m-d H:i:s");
        
        $query = mysqli_query($conn,"SELECT * FROM regist WHERE Username='$user' OR Mail='$mail'");
        if(mysqli_num_rows($query) > 0 ) {
          
          echo "<script type='text/javascript'>alert('User name or Email id already exists.');
               window.location='regist.html';history.back();
               </script>";
          }else{

          $sql = "INSERT INTO regist (Name, Username, Password, Mail, PhoneNumber, Sex, Religion, DateOfBirth, Question, Answer,date_time) 
            VALUES ('$name', '$user','$password','$email', '$phone', '$sex', '$nationality', '$dob', '$question', '$reply', '$insertdate')";

           }   
        

            mysqli_close($conn);

?>