How to reconnect to MQTT server after CONNECTIONLOST?

Question

I'm using a Raspberry Pi 3B+ with SIM7600X 4G HAT for communication with Python. I want to connect to MQTT server to receive messages via writing MQTT commands directly to the serial port of the SIM7600 (based on the manual examples).

It works fine for a while, but after a few hours it loses MQTT connection, and I can't reconnect the same way as I connected the first time.

This is my main loop:

def loop():
    global isMQTTServerReady
while True:
    if isMQTTServerReady:
        if ser.inWaiting():
            EVENT = ser.read(ser.inWaiting()).decode()

            if "+CMQTTCONNLOST" in EVENT:
                connectToMQTT()


            if "+CMQTTRXPAYLOAD" in EVENT:
                # do something with the MQTT message
    else:
        connectToMQTT()
    time.sleep(5)

And this is my connect function:

def connectToMQTT():
    openSerialPortIfNotOpen()
ser.write("AT+CRESET")
time.sleep(5)
log(ser.read(ser.inWaiting()))
time.sleep(1)

ser.write('AT+CMQTTSTART\r\n'.encode())
time.sleep(3)
log(ser.read(ser.inWaiting()))


ser.write('AT+CMQTTDISC=0,60\r\n'.encode())
time.sleep(2)
log(ser.read(ser.inWaiting()))

ser.write('AT+CMQTTREL=0\r\n'.encode())
time.sleep(2)
log(ser.read(ser.inWaiting()))

ser.write('AT+CMQTTSTOP\r\n'.encode())
time.sleep(2)
log(ser.read(ser.inWaiting()))
startAndSubscribe()


def startAndSubscribe():
    global isMQTTServerReady
    isMQTTServerReady=False
ser.write('AT+CMQTTSTART\r\n'.encode())
time.sleep(3)
log(ser.read(ser.inWaiting()))

ser.write('AT+CMQTTACCQ=0,'+MQTT_CLIENT_NAME+'\r\n'.encode())
time.sleep(5)
log(ser.read(ser.inWaiting()))

ser.write(MQTT_BROKER_URL+',60,1\r\n'.encode())

while not isMQTTServerReady:
    connStatus = ser.read(ser.inWaiting())
    log (connStatus.decode())

    if "+CMQTTCONNECT: 0,0" in connStatus.decode():
        time.sleep(.1)
        log(ser.read(ser.inWaiting()))
        ser.write('AT+CMQTTSUB=0,12,1,0\r\n'.encode())
        time.sleep(2)
        log(ser.read(ser.inWaiting()))
        ser.write(MQTT_TOPIC_NAME.encode())
        time.sleep(3)
        isMQTTServerReady=True

    if "+CMQTTCONNLOST:" in connStatus.decode():
        connectToMQTT()   

It works for a while, but after 1-2 hours I got into this loop (this is the log file):

AT+CRESET
AT+CMQTTSTART
ERROR
AT+CMQTTDISC=0,60
+CMQTTDISC: 0,11       #--> 11 means: No connection
ERROR
AT+CMQTTREL=0
+CMQTTREL: 0,19       #--> 19 means: Client is used
ERROR
AT+CMQTTSTOP
+CMQTTSTOP: 21       #--> 21 means: Client not released
ERROR
AT+CMQTTSTART
+CMQTTSTART: 23       #--> 23 means: Network is opened
ERROR
AT+CMQTTACCQ=0,"client_1"
+CMQTTACCQ: 0,19       #--> 19 means: Client is used
ERROR
AT+CMQTTCONNECT=0,"tcp://test.mosquitto.org:1883",60,1
+CMQTTCONNECT: 0,13       #--> 13 means: Not supported operations
ERROR

And I stuck in an infinite loop of trying to reconnect, but it can never reconnect again. But if I reboot the device, it works fine again.

What is wrong with this? I thought that if I lose connection, I should:

• disconnect client, -release client, -stop mqtt, -start mqtt, -acquire client, -connect to mqtt and then subscibe to topic, as it is in the Sim 7600X Manual