Attempt with tcp probe

gauge.py

@@ -884,12 +884,25 @@ def _subscribe_all(c):
         c.subscribe(f"{prefix}/status_led/green/set")
 
 
+def _probe_tcp(host, port, timeout_s=10):
+    """Open and immediately close a TCP connection to verify the route is up."""
+    import usocket
+    s = usocket.socket()
+    s.settimeout(timeout_s)
+    try:
+        s.connect(usocket.getaddrinfo(host, port)[0][-1])
+    finally:
+        s.close()
+
+
 def connect_mqtt():
     global client_ref, _mqtt_connected
     info(f"Connecting to MQTT broker {MQTT_BROKER}:{MQTT_PORT} ...")
     last_error = None
     for attempt in range(3):
         try:
+            _probe_tcp(MQTT_BROKER, MQTT_PORT)
+
             if client_ref is not None:
                 try:
                     client_ref.disconnect()