Please leave the interface on Auto Negotiate.
Did you test the WAN port too? Did it display 1Gbps? Your computer won’t get an IP address when directly connected to the WAN port, but it will display the link speed nonetheless.
Please also run this test: connect your old router to the modem, then connect the XR10 WAN to a LAN port on your old router. Connect your computer to the XR10 and run a speed test.
Also, what’s the model of your modem?