Checking if the data already exist in the wp database - html

Question

I'm attempting to display a message in red colour if the data already exists in the database but it doesn't seem to be working. This is the code that I'm using.
PHP code:
<?php
/**
* Template Name: Add IP Address
*/
get_header();
?>
<?php
$sql="SELECT * FROM `wp_5wbtdj_ip_based_login` where (start='$start' or end='$end');";
$res=mysqli_query($mysqli,$sql);
if (mysqli_num_rows($res) > 0) {
$row = mysqli_fetch_assoc($res);
if($end==isset($row['end']))
{
echo " End IP already exists";
}
if($start==isset($row['start']))
{
echo "Start IP already exists";
}
}
else{
//do your insert code here or do something (run your code)
}
If($_POST['Submit']){
global $wpdb;
$username = $_POST['username'];
$start = ip2long($_POST['start']);
$end = ip2long($_POST['end']);
If($wpdb ->insert('wp_5wbtdj_ip_based_login',
array(
'username' => $username,
'start' => $start,
'end' => $end
)
)== false) wp_die ('<div class="errormessage">Database insertion failed </div>');
else echo '<div class="successmessage"> Database insertion successfull </div>';
?>
<?php
}
else{
}
?>
HTML code:
<main role="main">
<div class="section section_white section_nopadding_top">
<div class="container">
<div class="row">
<div class="col-md-12">
<br><br>
<div class="wrap">
<form action="" id="postjob" method="post">
<h3 style="margin-left:50px;">IP Based Login</h3>
<hr />
<h4 style="margin-left:50px;">Add IP Range</h4>
<table class="form-table" style="margin-left:50px;">
<tr>
<th scope="row" valign="top">
<label for="username">Username</label><br />
</th>
<td>
<input type="text" size="25" name="username" value="<?php echo do_shortcode('[show_loggedin_as]'); ?>"><br /><br>
</td>
</tr>
<tr>
<div class="wrapper">
<th scope="row" valign="top">
<label for="start_ip">Start IP </label>
</th>
<td>
<input type="text" minlength="7" maxlength="15" size="25" pattern="^((\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.){3}(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])$" name="start" valign="top" required>
</div>
<br><br>
<tr>
<div class="wrapper">
<th scope="row" valign="top">
<label for="start_ip">End IP</label>
</th>
<td>
<input type="text" minlength="7" maxlength="15" size="25" pattern="^((\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.){3}(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])$" name="end" valign="top" required>
</div>
</td>
</tr>
<br>
</table><br />
<input name="Submit" class="button" value="Add IP range" type="Submit" style="margin-left:50px;" />
</form>
</div>
<br /><br />
</div> <!-- /col-md-12 -->
</div> <!-- /row -->
</div><!-- /.container -->
</div> <!-- /section -->
</main><!-- /.container -->
</div>
<?php
get_footer();
Can anyone help to point what I'm doing wrong here?