Hi,
On our devices ports are automatically 1000Mbps and cannot be changed, as this would require firmware and hardware changes. As I understand, it's probably your device that changes throughput from 1000Mbps to 100Mbps so you must set it on your device instead.
Also, try checking if your cable is at least CAT 5e to give out and receive full 1000Mbps transmit. It could also be that it's damaged and your software detects it as lower standard.
EB.