Lenovo ThinkVision E28u-20 28" 4K HDR Monitor - Manual (Page 11)

Rotate
Before the display is displayed in a vertical orientation, it should be fully raised
and fully tilted to avoid collisions.
Rotation
With the built-in base, you can tilt and rotate the monitor for the best viewing
angle.
Chapter 1. New Member Guide 1-4
Vertical
Rotation 90°
Horizontal
Rotation 360°