I am not from Intel, but I think you have described the expected behavior since that is not a supported OS for recent NUCs (they support Windows 7, 8.1, and 10). I am not certain but I believe that there may be some manual steps you could try to download/extract Intel's Pro1000 NDIS64 driver and probably modify some INF and then something like disable driver enforcement. Try web search using terms like Windows Server 2012 NUC.
Same problem here as well. Bought Windows Server 2012 R2 and installed it without problems (even no problem with UEFI). The devices are detected as 'Ethernet Adapter' and 'Network Adapter', but the corresponding downloaded LAN driver from Intel tells me 'no network hardware detected' and the WLAN driver installs, but nothing happens.
I cannot understand, that Intel is not supporting this OS, because it's similiar to Win8.1 and other drivers (Audio, Graphics,...) can be installed and are working fine.
Hope that Intel will quickly change this.
Otherwise I will try a workaround with an USB network adapter... but this sound so silly as the final solution on an up-to-date hardware with an up-to-date OS.
Thank you for joining the Intel communities.
Unfortunately Windows Server 2012-R2 has not been validated to work with this Intel® NUC and some drivers will not be available as you can see here:
Similar issue was reported at: