ok super j'ai réussi comme ceci.
if (!client.connect(server, 80)) {
Serial.println("erreur de connection");
Serial.println("attente x sec...");
delay(10);
//return;
}
// préparer la requete GET à ThingSpeak
String url = "/update?key=";
url += thingspeak_key;
url += "&field1=";
url += String(h);
url += "&field2=";
url += String(t);
// envoyer requete GET au serveur
client.print(String("GET ") + url + " HTTP/1.1\r\n" +
"Host: " + server + "\r\n" +
"Connection: close\r\n\r\n");
//lire la réponse ThingSpeak
String line = client.readStringUntil('\r');
client.println(line);
Serial.println("fermeture de la connection");
client.stop();
delay(15000);
}