Dell C7017T User Manual page 10

Operating range of the remote control
Point the top of the remote control toward the LCD monitor's remote sensor
during button operation.
Use the remote control within a distance of about 8 m from remote control
sensor or at a horizontal and vertical angle of within 15° within a distance of
about 5.6 m.
NOTE: The remote control may not function properly when the remote control
sensor on the display is under direct sunlight or strong illumination, or when there is
an obstacle in the path of signal transmission.
10 About Your Monitor