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Teltonika does not have an example specifically for interacting with Siemens S7 - 1200, but in the link below are given the instructions on how to set up your router as an MQTT broker:
You can find more information with MQTT configuration settings and their meaning in this wiki page: https://wiki.teltonika-networks.com/view/RUT955_MQTT.
A side note is that you might need to download an additional MQTT package to be able to configure MQTT broker. You can do it by accessing device's WebUI and navigating to Services -> Package manager.