fischertechnik community forum

On which device you think there is a firewall working?
RasPi, Siemens PLC, TXT, Router?

In the 192.168.0.xyz net there is no firewalling, encrypting, authenticating etc.

The RasPi (node-Red app) is acting as a bridge between Siemens PLC (OPC-UA port 4840) and TXT controller (MQTT - port 1883).
OPC UA communication between PLC and RasPi via port 4840 is unrestricted and could be observed with e.g UA-Expert client.
MQTT between Raspi and TXT is also unrestricted. You can use e.g. MQTT-Explorer to see the MQTT traffic.

The communication between TXT and ft-cloud might be secured in some way - this is what I do not know.

Maybe you can describe what is your problem exactly - then someone could provide you with more exact information...

With MQTT you mix up some things.

MQTT is a communication between clients and a broker and not between devices.
The clients are embedded in the applications.
A client can have a subscriber and/or a publisher relation with a broker.
The broker configuration is responsible for the ports (default 1883) which are used, the security (id/psw's and ACL (for the topics)).
Brokers can be connected with each other by bridges.