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fix Ethernet
Download IntelMausiEthernet.kext from github or onedrive , copy the kext file to SSD's EFI/CLover/Other and Ethernet should works on next boot.
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WIFI
This motherboard(Gigabyte Z390 I WIFI) has builtin INTEL wireless card. But it doesn't work in macOS, the the M.2 slot which the wireless card plugged in only support CNVi devices, there is no CNVi devices compatiple with macOS right now. So if you want a Wifi, you need a PCIe wireless card or a USB device. I prefer reverve the only PCIe slot for eGPU and buy a USB wireless device: Asus AC53 Nano, to make it works just download OSX driver from asus's website and install.