Hello @sbr,
the IOs of the Neuron are not connected to the GPIOs. Instead, they are accessible via local ModbusTCP server (you can see the Modbus Registers Map)

Or better yet, check the official Neuron plugin for Codesys: https://store.codesys.com/unipi-neuron-for-codesys-sl.html?___store=en&___from_store=default

Best regards,
Martin