is_single doesnt work for custom post type

<!-- Inner Pages Main Section -->
<section class="ulockd-service-details">
    <div class="container">
        <?php
        if ( is_single('2631') == true ) {

            echo "2631";

            echo " <div class="col-md-4 col-lg-3"> ";

custom page id is 2631 but echo doesnt display.

look at this link :

https://resimli.yedek.deniz-tasarim.site/services/left-sidebar-service/

https://resimli.yedek.deniz-tasarim.site/services/left-sidebar-service/?preview_id=2631&preview_nonce=4df87aba10&preview=true&_thumbnail_id=364