Providers and slots GEO query WordPress.

test.php

<?php

$providers_query = new WP_Query(
	array(
		'post_type' => 'provider',
		'tax_query' => array(
			array(
				'taxonomy' => 'country',
				'field'    => 'slug',
				'terms'    => 'nl',
			),
		),
	)
);

$provider_slugs = wp_list_pluck( $providers_query->posts, 'post_name' );

$slots_query = new WP_Query(
	array(
		'post_type' => 'slot',
		'tax_query' => array(
			array(
				'taxonomy' => 'game_provider',
				'field'    => 'slug',
				'terms'    => $provider_slugs,
			),
		),
	)
);