functions.php

add_filter( 'updated_users_subscriptions_for_order', 'wc_subs_suspend_on_cod' );
function wc_subs_suspend_on_cod( $order ) {

    if ( ! is_object( $order ) ) {
        $order = new WC_Order( $order );
    }

    foreach ( WC_Subscriptions_Order::get_recurring_items( $order ) as $order_item ) {

        $subscription_key = WC_Subscriptions_Manager::get_subscription_key( $order->id, WC_Subscriptions_Order::get_items_product_id( $order_item ) );
        $payment_gateways = WC()->payment_gateways->payment_gateways();
        $payment_gateway  = isset( $payment_gateways[ $order->recurring_payment_method ] ) ? $payment_gateways[ $order->recurring_payment_method ] : '';

        if ( $payment_gateway->id == 'cod' && $order->get_status() != 'completed' ) {
            WC_Subscriptions_Manager::put_subscription_on_hold( $order->user_id, $subscription_key );
        }
    }
}