Set a variable in an ISY994i from a device that sends MQTT messages

[OldSurferDude]

I'm looking for documentation, a recipe, perhaps, that shows my how to set a variable in an ISY994i from a device that sends MQTT messages.

Polyglot v2.2.11 fev2.2.9-5

My MQTT client is an Arduino and the library is necessarily small. It does fine sending and receiving messages through a broker.

My Polyglot box is HP x64 16GB ram, 1T disk, Ubuntu 20. On it I am running an MQTT-broker and because of that I set the port of Polyglot 1884. I start a subscriber with mosquitto_pub -t test . Then I send a message with mosquitto_pub -t test -m "test message" and this works as expected. But when I add in the -p 1884 option to both, pub responds with Error: The connection was lost.

So where is the documentation that 1) explains how to send a message to Polyglot and 2) to what topic is my ISY994i subscribed and in what format should the message have, so that I can set a variable?