Disable Wi-Fi when LAN cable is plugged in and enable it when the cable is plugged out

99-disable-wireless-when-wired

#!/bin/sh

IFACE="${1}"
ACTION="${2}"

ntfy () {
  LOGGED_USERS=$(users)

  for user in ${LOGGED_USERS}
  do
    USERID=$(id -u ${user})
    sudo -u ${user} \
      DISPLAY=:0 \
      DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/${USERID}/bus \
      notify-send "${1}" "${2}"
  done
}

case ${IFACE} in
  eth*|usb*|en* )
    case ${ACTION} in
      up )
        ntfy "Ethernet connected" "Disabling Wi-Fi"
        nmcli radio wifi off
        ;;
      down )
        ntfy "Ethernet disconnected" "Enabling Wi-Fi"
        nmcli radio wifi on
        ;;
    esac
    ;;
esac

This gist is based on https://gist.github.com/cengizIO/c9023b83a8590dd2c723f3181633c99a

The script should be placed under /etc/NetworkManager/dispatcher.d/ with a name like 99-disable-wireless-when-wired.
Make sure that it's owned by root and that it's executable (sudo chown root:root /etc/NetworkManager/dispatcher.d/99-disable-wireless-when-wired && sudo chmod +x /etc/NetworkManager/dispatcher.d/99-disable-wireless-when-wired).
Also, NetworkManager-dispatcher service should be enabled and running for this to work.