Your homebrewed software capture does not show any TCP packets. Your software is not establishing a connection and is not communicating with the MC 750 at all.
You need to rewrite your homebrewed software. The .NET library I sent a link for is one way you could do this, and is likely the easiest method, since it does not require you to know the Modbus protocol in great detail. Another popular .NET library you could use is NModbus https://github.com/NModbus/NModbus
You need to rewrite your homebrewed software. The .NET library I sent a link for is one way you could do this, and is likely the easiest method, since it does not require you to know the Modbus protocol in great detail. Another popular .NET library you could use is NModbus https://github.com/NModbus/NModbus