Communication over web Unity, pi, mqtt, node-red, mySql

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I want to make a communication between plc / pie and a pie over the internet, I will set up a mysql database and a node-red on the pie that is not by plc. and from the same pie I want to send graphs as well as start / stop of functions to a unity app. The idea was to use the pie as a vpn and communicate with mqtt but did not make this work. anyone who can be helpful with clues? thanks in advance