Enable on Checkout Cross-sells

Thanks for the suggestion @goferit! The free version of this plugin only modifies the links on single product pages. Shop pages, categories and other product listings (including related products/cross sells), are only covered in the Pro version.
If you want to prevent customers from exiting your checkout for external products, you may want to disable the suggested products altogether, there’s a tutorial for that on businessbloomer.com

Additionally/alternatively, if you only want to automatically remove the external products from the cross sells completely, you can do that with the following code snippet:

add_filter('woocommerce_cart_crosssell_ids',function($cross_sells,$cart){
	foreach ($cross_sells as $key => $product_id) {
		if ($product = wc_get_product($product_id)) {
			// remove if external
			if ( $product->is_type( 'external' ) ) {
				unset($cross_sells[$key]);
			}
		}
		return $cross_sells;
	}
}, 10, 2 );

If you haven’t worked with code snippets (actions/filters) or functions.php before, read this guide: How to use filters

Hope that helps, let us know if you have any further questions!