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The Controllino Maxi has its RS485 interface already wired (with no termination inside) and the driver is connected to the UART3 (TxD3 / RxD3) of the ATmega2560.
so if you have the right cable then you need wires coming into the Controllino Maxi form your sub DB9 pins 2,3 and 5. Connect pin 5 to GND of the Controllino Maxi RS485, pin 2 probably to - and pin 3 to +
(verify polarity). I assume they come from the remote connector pin 5/6/7.
then try writing a piece of code listening to what's coming into Serial3