The Ethernet adapter (based on I219-V) does not seem to support 1 Gbps, only 100/10 Mbps. Even with Cat6 cable and sub-1m connection to any other device can only be performed at 100 Mbps, other devices in the same configuration support 1 Gbps without any problem.
The BIOS settings don't offer 1 Gbps, and the firmware is only version 0.0.19, which suggest it's alpha. Yet I have NUC BIOS version 74 (BECFL357.86A.0074), which is the latest available.
This suggests the Ethernet component is unstable, or that the BIOS is not final and perhaps not configuring the device optimally. Please let me know when you plan to have an update for the adapter firmware.
This has been tested with different versions of Linux including openSUSE TW. If you are currently not supporting Linux, do you have any plan to do so? What would you advise regarding the drivers?
As you know, Linux is not supported. Future planned support cannot be answered here.
I just setup a NUC8i7BEH for a friend on Windows 10, with cat6 cable directly to the router, and all worked as it should.
You may want to verify/replace your cable. I had a cat 6 cable that was defective and only did 10/100. Also, if there are any switches in your mix, verify that they are 10/100/1000 capable.