Hello,
To answer your question:
Do I need to configure something to be able to write the Modbus registers in the RUT200?
Shortly, yes, you need to have the Modbus TCP Server (previously referred to as slave) enabled on the RUT200 in order to write to its registers. Without the Modbus TCP Server active, the device will not accept write commands.
Additionally, there is a wiki documentation guide below describing the Modbus TCP Server SMS sending setup, which I believe matches the configuration you are trying to achieve:
You may also find it helpful to review the related community thread discussing a similar SMS sending over Modbus TCP scenario here:
Best regards,