PHP Session data not saving over AJAX call [closed]

Its an AJAX login script

HTML:

<form class="ajax-submit" method="POST" action="https://example.com/api/accounts">
     <div class="form-group">
           <input type="text" class="form-control" name="uname" placeholder="Username or Email" value="">
     </div>
     <div class="form-group">
           <input type="password" class="form-control" name="password" placeholder="Password" value="">
     </div>
     <div class="form-group">
           <button class="btn btn-primary btn-block" type="submit">
                                Login
           </button>
     </div>
</form>

Nothing interesting here really, its just a form that gets submitted over AJAX.
This is the action file (https://example.com/api/accounts):

if($DB->AuthUser($email, $password)){
   $success = true;
}else{
   $success = false;
}

if($success){
   $json->success = true;
   $json->redirect = "https://example.com/?in-development";
}else{
   $json->success = false;
   $json->messages->password = "Wrong email or password";
}

As you can see, the session data is set in the DB class ($DB->AuthUser function) which is a third file (don't know if this can cause something).

public function AuthUser($field, $password)
{
    session_start();
    $stmt = $this->conn->prepare("SELECT * FROM user_profiles WHERE (`email` = ? OR `username` = ?) LIMIT 1"); 
    $stmt->execute(array($field, $field)); 
    $row = $stmt->fetch();
    if(empty($row))
    {
        return false;
    }

    if(password_verify($password, $row['password'])){
        //set session data
        $_SESSION['user'] = $row;
        return true;
    }else{
        return false;
    }
}

Here is the JS:

var formData = new FormData(this);
    $.ajax({
        url: $(this).attr('action'), 
        type: 'POST',
        data: formData, 
        cache: false,
        processData: false,
        contentType: false,
        dataType: 'json',
        success: (function( data ) { ... }

Everything works, except session data not saving over the request (it does save without the AJAX request, but I don't want to drop it)