Custom post-type’s pagination not working in category.php

The issue is that pagination is based on the main query. Whether or not pages exist is determined by the query that is run before the template is loaded. When you create a new query in the template, it’s not connected in any way to that default main query.

The proper way to alter the main query is via the pre_get_posts action:

function wpa_custom_post_types_category( $query ) {
    if ( $query->is_category() && $query->is_main_query() ) {
        $query->set( 'post_type', array( 'post', 'articulo' ) );
    }
}
add_action( 'pre_get_posts', 'wpa_custom_post_types_category' );