i also had big problems until i got it work (migrated from mqtt1 to mqtt2,4). as already mentioned i think it is important not only to install the mqtt binding, but even the “embedded mqtt broker” (to be found under misc). but then you must not have an external mqtt-client installed because they will not work parallel.
for me this was helpful:
sorry when i cannot help more, i´m a pure beginner and lucky because today got mqtt2.4 working finally