1. Please check the system BIOS to verify if the 'USB Ports' are 'enabled'. You may find the following steps useful:
- Power off the laptop > Power it back on and at 'Dell splash screen , repeatedly tap 'F2 key' > Use 'arrow keys' on the keyboard to navigate to the option 'Advanced' > Navigate to 'Miscellaneous Devices' > 'Enable USB' > Tap 'F10 key' to 'Save Settings' (Refer to the images below)
Alternatively, you may tap 'F9 key' to 'Load BIOS defaults' and 'F10 key' to 'Save Settings & Exit'
2. 'XPS 15z/L511z' has 2 'USB 3.0 ports' on the 'left side' (Numbered 4 in the image)
If the issue persists, it is likely that 'USB 3.0 Host Controller' is missing. Use the following link to install it and check if the ports work: Driver Details | Dell United States
Hope this helps. Do reply if you have any further questions. I would be glad to assist.