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Please go to the system BIOS> System Configuration> USB/Thunderbolt Configuration. Please place the check mark for the below,
Enable USB Boot Support
Enable Thunderbolt ports
Always Allow Dell Docks
Enable External USB port
Then you may first try with Security level – No Security, if the issue persist try Security level – Display Port Only.
Also please the to the monitor settings and turn the Deep sleep mode off.
Kindly get back if there are any further queries.
For me to better assist in troubleshooting, please click my DELL-username and send me a Private Message with the Dell Service Tag number and your email address.