Ethernet port not working

Seems like it's fixable...

If you have the Incompatible Kernel Extension Configuration Data 3.28.1 update installed, but do not yet have the Incompatible Kernel Extension Configuration Data 3.28.2 update installed, the Ethernet drivers for your Mac will be blocked from running the next time the Mac is restarted. That will result in your Mac not being able to connect to your network via an Ethernet connection.

To fix this, the Incompatible Kernel Extension Configuration Data 3.28.2 update needs to be installed and the Mac rebooted again. After this reboot, the Ethernet drivers should be enabled again and your Mac’s Ethernet connection should work normally again.

To install the Incompatible Kernel Extension Configuration Data 3.28.2 update:

1. Verify you have a working connection to the Internet
2. Open Terminal
3. Run the following command with root privileges:

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softwareupdate --background-critical

Running the softwareupdate –background-critical command forces a check-in with Apple’s software update service and will trigger your Mac to automatically install the Incompatible Kernel Extension Configuration Data 3.28.2 update.

For those interested in checking the affected kernel extension blacklist, it is stored in the following location:

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/System/Library/Extensions/AppleKextExcludeList.kext/Contents/Info.plist

When the Incompatible Kernel Extension Configuration Data 3.28.1 update is installed, the /System/Library/Extensions/AppleKextExcludeList.kext/Contents/Info.plist file is updated with the following entries, which add Apple’s Ethernet drivers to the kernel extension blacklist:

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<key>com.apple.iokit.AppleBCM5701Ethernet</key>

<string>LT 10.2.0</string>

<key>com.apple.iokit.AppleYukon2</key>

<string>LT 4.0.0</string>

When the Incompatible Kernel Extension Configuration Data 3.28.2 update is installed, those entries are removed:

Can you see if your plist has these lines removed?

<key>com.apple.iokit.AppleBCM5701Ethernet</key>

<string>LT 10.2.0</string>

<key>com.apple.iokit.AppleYukon2</key>

<string>LT 4.0.0</string>

/System/Library/Extensions/AppleKextExcludeList.kext/Contents/Info.plist

Apple security update blocks Apple Ethernet drivers on OS X El Capitan...

https://derflounder.wordpress.com/2016/02/28/apple-security-update-blocks-apple-ethernet-drivers-on-el-capitan/#:~:text=To%20fix%20this%2C%20the%20Incompatible,connection%20should%20work%20normally%20again.