24V to 5V converter using a LM3489
I verified all my circuit but nothing is wrong


I beg to differ.....

Using a 0.1 uF capacitor across the feedback resistor is asking for trouble. The data sheet says that it should be 100 pF i.e. 1000 times smaller than what you have. The output is probably going unstable and oscillating and your meter is averaging the output and giving you a number that you think means it has 1.8 volts DC. Your output capacitor is also missing: -




  Is the LM3489 a good solution ?


I think the FDC5614P is a little too lightweight for a 2.1 A output - how are you heatsinking this tiny little device? Note that the application diagram in my answer uses the same FET but the output is only rated for 0.5 amps.