Ethernet and Thunderbolt don't work on MacBook Air

It can sometimes take a minute or two before an Ethernet connection will be established.

Try booting into Safe Mode to see if that makes any difference. If this works, then most likely you have some third party software installed which is interfering with the normal operation of macOS. Usual culprits are anti-virus apps, cleaning/optimizer apps, third party security software, and VPN software.

Try booting into the Apple Diagnostics using the Ethernet adapter. This can help eliminate any OS issues affecting the connection.

Make sure the contacts in the Thunderbolt port are clean. In supporting thousands of my organization's Mac, I find the contacts in these ports tend to get a lot of grime on them and even liquid damage. Some devices may not be utilizing all the pins on the port. If you need to clean the port, then use a wooden or plastic toothpick to clean the contacts making sure to only move the toothpick from the back of the contact to the front. Do not move the toothpick side to side or pushing the toothpick front to back or you may damage/bend the contacts.