If you are installing an Ethernet cable in a busy environment with lots of electronic devices, wireless signals, or leaky microwaves, then make sure to investigate Ethernet cables with additional shielding.

Source: https://www.digitaltrends.com/computing/different-types-of-ethernet-cables-explained/

If your Mac does not have an Ethernet port, you can use an adapter to connect the Ethernet cable to the USB or Thunderbolt port on your Mac.

Source: https://www.devicemag.com/data-for-macbook/

One possibility is that the Ethernet cable connecting the Airport Extreme to the modem or router may not be properly connected.

Source: https://www.devicemag.com/airport-extreme-internet-connection-problem/

One way to get around this issue is to invest in a streaming device that can be directly connected to the internet via an Ethernet cable.

Source: https://www.cnet.com/tech/home-entertainment/best-wired-streaming-device/

To access a network via an Ethernet connection, you need to connect your device to the network via Ethernet cable.

Source: https://www.devicemag.com/connect-mac-to-router-with-ethernet-cable/