Hello @gtecaskari, hope you're doing well.
Could you please share which wifi example and SDK version are you working with?
Additionally, are you looking forward to enter a certification process? This is important since that API is meant to work with dBm ranges from 0 to 24.
Regarding the change of tx power in AP mode I suggest to check section 3.1.7 of the NXP Wi-Fi and Bluetooth Demo Applications for FRDM-RW61x, where a guide to add custom commands to a wifi command line is provided, in this way you can be able to add a command to change the transmit power while being able to get into AP mode within the same sample.
Best Regards!