In my S9 , Power Mode .>>> Max power saving is selected by default and cannot change to high performance, optimized or medium power saving option. Adaptive power saving is off by default.
When selecting any other options, the screen shows changes in brigtness in %, screen resolution then after applying shows turning on and progress moves to 100%. No changes happens in phone, changes back to maximum power saving. Due to this problem, many options in phone will not work including wifi file transfer.
Have you tried to reset your settings?
This device allows you to reset the settings to their defaults without doing a full factory reset. Any phone customizations will be discarded but personal data and settings of downloaded apps will not be affected.
Would you mind providing your full model code and Android version so I can look up device appropriate troubleshooting steps?