nickbaak:
Hello and welcome to the forums.
Features and functions supported by a particular hardware device are most often a product of the software driver for the device, so....
Determine the Manufacturer, Model Name, Model Number, Revision Number of your network adapter and use the manufacturer's website to obtain the latest driver for your ethernet card and see if that adds a more robust feature set which will support changing the link speed and duplex as you wish.
Thanks for joining our community and keep us posted.
Regards
Randy