functions.php

<?php

// Replace the Add to Cart Button on Single Product Template to go to a link that is also an ACF plugin URL field
add_action( 'woocommerce_single_product_summary', 'replace_single_add_to_cart_button', 1 );
function custom_product_button(){
	$product = wc_get_product();
    // HERE your custom button text and link
    $button_text = __( "Buy from external resource", "custom-textdomain" );
    
    // Display button
    echo '<a class="button" style="text-align: center; margin-bottom: 10px;" target="_blank" href="'.get_field('external_product_link', $product->get_id()).'">' . $button_text . '</a>';
}

function replace_single_add_to_cart_button() {
	// Here we completely get rid of Add to Cart button and change it to an external button	
	remove_action( 'woocommerce_single_product_summary', 'woocommerce_template_single_add_to_cart', 30 );
	add_action( 'woocommerce_single_product_summary', 'custom_product_button', 30 );	
}