Increase the post query limit for the Output tab setting by uploading this file as a plugin.

optin-monster-query-limit.php

<?php
/**
* Plugin Name: OptinMonster Query Limit
* Description: Increases the default query limit for page-level targeting.
* Version: 1.0.0
*/

add_filter( 'optin_monster_post_selection_limit', 'tgm_om_post_selection_limit' );
function tgm_om_post_selection_limit( $limit ) {
    
    return 9999;
    
}