Just a quick reminder to my self in case I make this mistake in the future. The configuration of the built-in ethernet port open settings in GX Developer IEC requires port numbers to be expressed in hex.

This can be confusing if the configured port number only use digits between 0 and 9 like my current configuration. I managed to get confused by this even though the dialog explicitly states that you need to use hexadecimal numbers for the port configuration.

If you take a quick look at it and try to send a request to port 1392 you will not succeed:

client.Open("192.168.0.1", 1392);
client.ReadWords("D0", 1); 

The correct code also needs to use the hexadecimal number:

client.Open("192.168.0.1", 0x1392);

Or use the correct decimal number:

client.Open("192.168.0.1", 5010);