MQTT no "topic"

In the QNAP VM I installed HA-Suppervisor, MQTT and MQTT-Explorer. Configured =
Server: core-mosquitto
Port: 1883
mqttuser+Pw from settings.
Publishing and receiving a package works.

In MQTT Explorer, configured with the IP address of the HA,
mqttuser+Pw the connection works,
but I only see the IP → $SYS → broker and values ​​but no topic.

What am I doing wrong?