How to reduce transmission TX power

My router is an Asus RT-AC56U using the current Asuswrt-Merlin firmware. My only wireless device is 2.4GHz only and is close to the router.

Changing Tx power adjustment to 1% does nothing.

What is the custom code to change the 2.4GHz band to 10% and to change the 5GHz band to 0% (or off)?

And where do I enter the custom code?

Thanks!