Hi Jun,
The JN5168-001-M03 is not a high power module, if you try to enable it using the function you describe, you'll get an error because there's no power amplifier in the module you're using:

Only the JN5168-001-M05 or M06 modules are high power capable (regarding the JN5168 variant of MCUs), so this function is only compatible with this specific variant. I have attached a document to show how to enable high power mode on the compatible modules.
I can give you the following recommendations:
- I would recommend you try changing your network's channel to see if the low range is related to noise in the current frequency you're operating.
- You could also try adding an external Power Amplifier between the module's output and the antenna.
- I would also advise that you check the matching circuit of the module.
Could you please try to see if these recommendations work for you?
Please let me know if you have any other questions.
Best regards,
Sebastian