ShellyPlusHT + mqtt issue

Is there any special thing about mqtt in connection with shelly gen2 devices?
I already struggled somewhat with the web socket integration dueto my SSL certificate, but finally it is working,
Just for learning purposes I tried mqtt Integration, but I could not get it to work.
In the web interface of the shelly the mqtt state always shows ‘not connected’, but
in mqtt-Explorer I see my shellyplusht as online, but I don’t see any further topics.
What can I do to analyze the problem? Any ideas?