Wordpress - if (is_page(**PAGE ID**)) not working

you can use this for

<?php 
global $post;

if( $post->ID == 346) { ?>

      <!-- do your stuff here -->

<?php } ?>

you can use this anywhere either in header or anywhere else.

A simpler solution will be to pass the title or the slug as argument in is_page(). You won't have issues if you duplicate that page on another server.

<?php
if (is_page( 'Page Title' ) ):
  # Do your stuff
endif;
?>

Hooks such as init will not work at all.

You have to hook at least on parse_query.

Everything bellow will work:

is_page(198); # ID (int)
is_page('198'); # ID (string)
is_page('Some Title'); # Title, case-sensitive
is_page('some-title'); # Slug

But it must be hooked at least in parse_query or any other hook after it. You can see WordPress hook order here: https://codex.wordpress.org/Plugin_API/Action_Reference