Template tags to display custom post type posts in category template?

Custom post types are by default excluded from the main query except on taxonomy pages and custom post type archives.

You can simply use pre_get_posts to correctly alter the main query (alter the query variables before the SQL query is build and executed) to your needs.

Just a few notes on pre_get_posts

• pre_get_posts runs front end and back end queries, so it is very important to do the is_admin() check for queries only meant to run front end or back end

• pre_get_posts alters all custom instances of WP_Query, get_posts() (which uses WP_Query) and the main query (which also uses WP_Query). You would want to use the is_main_query() check to specifically only alter the main query.

You can do the following in a plugin or your theme’s functions.php

add_action( 'pre_get_posts', function ( $q )
{
    if (    !is_admin() // Only target front end queries
          && $q->is_main_query() // Only target the main query
         && $q->is_category() // Only target category archives
    ) {
        $q->set( 'post_type', ['post', 'custom_post_type'] ); // Adjust as needed
    }
)};