I next connected to the device (which is successfully online in internet mode) to xxxxxxxx from https://flow.m5stack.com/

I then made a minimal MQTT connect and publish program to a public test MQTT broker and published a message.

from m5stack import *
from m5ui import *
from uiflow import *
from m5mqtt import M5mqtt

setScreenColor(0x222222)

m5mqtt = M5mqtt('kilnageer', 'test.mosquitto.org', 1883, '', '', 300)
m5mqtt.start()
m5mqtt.publish(str('kilnageer/test'), str('hello'), 0)

I tested the public broker with the following code..

pi@tola-smarthome-v3:~ $ mosquitto_sub -h test.mosquitto.org -t kilnageer/test &
1 9842
pi@tola-smarthome-v3:~ $
pi@tola-smarthome-v3:~ $ mosquitto_pub -h test.mosquitto.org -t kilnageer/test -m "hi"
pi@tola-smarthome-v3:~ $ hi

pi@tola-smarthome-v3:~ $

pi@tola-smarthome-v3:~ $ hello

pi@tola-smarthome-v3:~ $
pi@tola-smarthome-v3:~ $ date
Mon 28 Nov 09:50:57 GMT 2022
pi@tola-smarthome-v3:~ $

So seeing "hello" proves the running the code works..

Also running a web terminal to the device at this time showed no errors:

Next I downloaded the code (main.m5f) to the device and so changed it from "internet" mode to "app" mode.

Now I now longer see the message published and the terminal shows and MQTT error:

APIKEY: xxxxxxxx
MicroPython 77320d1de-dirty on 2022-11-24; M5Stack with ESP32
Type "help()" for more information.
>>> [2495] MQTTException -> 128
reconnect start ...
reconnect finish ...

Also when I was developing the real code and connecting to a local Pi based MQTT broker I could see that the connection was never made (in the failing case) by looking at the mosquitto logs.

pi@tola-smarthome-v3:~ $ sudo tail -f /var/log/mosquitto/mosquitto.log
1669623877: New connection from 192.168.3.15 on port 1883.
1669623877: New client connected from 192.168.3.15 as mqttdash-10aaf3d5 (c1, k30, u'tola').
1669624454: Socket error on client mqttdash-10aaf3d5, disconnecting.

But I could see the connecting being made by the M5Stack client when I just ran the code from the web console.