Quicklaunch Action Catalog has a list of commands to control Dell C5518QT and C5522QT monitors via serial communication. Make sure you are using a proper cable to connect the monitor to your USB-Serial converter.
The Dell C5518QT and C5522QT monitors both require that the RS232 cable has pin 3 as TXD. Pin 3 refers to the pin in the center of the DB9 cable.
Both Monitors have a Male RS232 port, if you have a USB-RS232Male cable, you will require a DB9 Female-Female adapter
The C5518QT also requires that the Ethernet connection mode be disabled before it will listen to RS232 commands.