Resolution of monitor using Nvidia graphics driver with Ubuntu 16.04

When I try to mirror an external monitor with my computer, the highest resolution offered in Display Settings is 1024x768. However, both monitors should allow a resolution of 1920x1080. I suspect this is a compatibility issue between the Nvidia graphics driver and Ubuntu 16.04. Other suggestions using “xrandr” have not worked, probably due to the proprietary nature of the graphics driver. Can anyone help?

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