Here are some other items you can try:
- Check the System Log (logread) on the GL-MT1300, just after you plug in the LAN port, for any errors or unusual messages.
- Try changing to a different CAT5e or better Ethernet cable.
- Try manually setting a static IP address on the Beryl, instead of using DHCP, to see if it communicates.
- Try plugging in a different computer, tablet or other device with a 1000Mbps Ethernet port and that uses DHCP to see if it gets an IP address.