Custom Category Template for Custom Post Type

Created a custom post type. It works.

Added custom categories to it. They work.

The archive itself works with the link:
https://domain.com/my_customposts/

Since I created:
archive-my_customposts.php

Tried created:
taxonomy-custom_categories.php
category-custom_categories.php

And even:
taxonomy-my_customposts.php
category-my_customposts.php

Using the custom category link inside the Dashboard that looks like this: https://domain.com/custom_categories/category1/

I get:
Error 404 Not Found

The code in functions is as follows:

add_action( 'init', 'create_post_type' );
function create_post_type() {
	register_post_type( 'my_customposts',
		array(
			'labels' => array(
				'name' => __( 'Custom Posts' ),
				'singular_name' => __( 'Custom Post' )
			),
		'public' => true,
		'has_archive' => true,
		)
	);
}

add_action( 'init', 'build_taxonomies', 0 );
function build_taxonomies() {
    register_taxonomy( 'custom_categories', 'my_customposts', array( 'hierarchical' => true, 'label' => 'Custom Categories', 'query_var' => true, 'rewrite' => true ) );
}

What am I missing?