Using a custom hooked function in woocommerce_order_details_after_order_table
action hook in which we add the woocommerce_thankyou_{$order->get_payment_method()}
hook, will do the job:
add_action( 'woocommerce_order_details_after_order_table', 'view_order_custom_payment_instruction', 5, 1); // Email notifications
function view_order_custom_payment_instruction( $order ){
// Only for "on-hold" and "processing" order statuses and on 'view-order' page
if( in_array( $order->get_status(), array( 'on-hold', 'processing' ) ) && is_wc_endpoint_url( 'view-order' ) ){
// The "Payment instructions" will be displayed with that:
do_action( 'woocommerce_thankyou_' . $order->get_payment_method(), $order->get_id() );
}
}
Code goes in function.php file of your active child theme (or theme) or also in any plugin file.
Tested and works in WooCommerce 3+.