Using branded factory-terminated CAT6 or self-clamp CAT6?

Check your router settings whether that port is set to auto or 100 mbps. If auto or 100 mbps, try change it to 1000 mbps. If router's port setting is correct and you still have the issue of 100 mbps connection. Try to check your NIC for your desktop or laptop, set it manually to 1000 mbps. It should immediately take effect of your changes. If not, try reboot your laptop/desktop. If still got problem, change the cable with new CAT6 cable and see.

Sometime after upgrade router/switch firmware, it will auto downgrade the speed to 100 mbps. You need to set it back to auto or 1000 mbps.

Suspect the router settings. Try reset it and reconfigure it step by step.

Still under warranty? If yes, send it for warranty claims.