The wiki has a useful generic step-by-step guide to implement MQTT client.
However, following the steps described there I was unable to connect an Arduino Nano 33 IoT to OpenRemote using MQTT. I have also addressed firewall issues creating rules for ports 1883 and 8883 and used the command shown here and here to lauch OpenRemote:
cmd /C “set KEYCLOAK_FRONTEND_URL=https://192.168.1.5/auth && docker-compose -p openremote up”
Note that in the tutorial kindly shared by Tomas above, I was not able to connect MQTT Explorer to OpenRemote using port 1883. It would be useful to know how this could be achieved.
After a lot of reading and digging, I could not find a single example of a successful implementation of a MQTT device on Arduino with complete code.
I suspect it would not be too difficult for someone with more experience to create a couple of examples of a real MQTT implementation on a widelly used platform such as the Arduino.
I would suggest that, if there is anyone with the required experience, to create simple examples using the Nano 33 Iot (and perhaps also the ESP32, just to cover two different wifi libraries), using ports 1883 and 8883.