ethernet port dead?

Try resetting the SMC How to reset the System Management Controller (SMC) on your Mac - Apple Support

and NVRAM Reset NVRAM or PRAM on your Mac - Apple Support

There are details in several support articles that could help resolve this matter.

One issue may be due to some failure of the system to see a proper connection.

How to use network locations on your Mac - Apple Support

Choose Apple menu > System Preferences, ... If you're using network locations ...

Your Mac automatically searches these services for a network or Internet connection.

https://support.apple.com/en-us/HT202480

To select from Network preferences and choose Ethernet (drag it to top of list) is

one way; because usually macOS or OS X will try the Service Order, as a priority.

(A 'network diagnostic' tool is also provided in the macOS.)

There are times when the ports attached to the logic board may fail.

So that may imply a course of action to have them tested or repaired.

However your older MacBook is newer than mine; Ethernet is essential.

At least for wired faster speed internet and local/distant networking, etc.

Wonder if this would work on your model?

Apple USB Ethernet Adapter - Apple

The cost to repair (depending on what the cause of failure is) may be

equal to some 'other like-model' that has seen repairs already. Better

'independent authorized' service/resellers can repair older models.

In 2016, users found a system update caused their Ethernet to fail. In

both iMac & MacBook Pro; a workaround suggested here may work.

http://www.idownloadblog.com/2016/02/27/security-update-disables-ethernet-on-som e-macs/