Is the HMI program using Modbus function 05 to write to coil 258? Only function 01, 02, 03, 04, 05, 06, 16 are supported.
I tried using a Modbus Poll master to send to output coil 258 and output #2 is turned ON.
Also ensure that the coil or relay are not already controlled by your PLC program. Otherwise what you manage to do is to change it for one program scan time (which may be < 1ms) and then your PLC program change it back to what the ladder logic or TBASIC decides the coil status should be.
The best way of testing is to put a blank program into the PLC and then try using your HMI to toggle a coil and see if it works with the PLC.