The S9 and S9 Plus’ max screen resolution is 2,960×1,440 pixels (also known as WQHD+), but did you know that the phone defaults to 2,220×1,080 pixels (FHD+)? The more conservative setting preserves battery life, but for the maximum detail, you can adjust the setting up or even down to 1,480×720 pixels (HD+).
Go into Settings → Display → Screen resolution