Monitor Connected Devices to an ASUS Router Using Raspberry Pi https://blog.1a23.com/2020/03/03/monitor-connected-devices-to-an-asus-router-using-raspberry-pi/

wlan_monitor.py

import subprocess
import requests

# MAC address of device running this script
self_mac = "00:11:22:33:44:55"

# MAC address of devices to check
to_monitor = {"66:77:88:99:aa:bb", "cc:dd:ee:ff:00:11"}

# MAC address of device which must be in the LAN for notifications to be sent
i_mac = "22:33:44:55:66:77"

# Path to the status file
record = "/etc/wlan_mon_status"

# Content of the status file
NOTIFIED_FLAG = "notified"


# Read in status file
with open(record) as f:
    data = f.read()

# Output of ARP table check
out = subprocess.getoutput('ssh admin@192.168.1.1 /sbin/arp -n').lower()

if self_mac not in out:
    # Error occurred while connecting to router
    print("err", out)
    exit(1)

if all(i not in out for i in to_monitor):
    # All devices to check is disconnected
    if NOTIFIED_FLAG not in data:
        if i_mac in out:
            # only notify if my device is still connected
            # Send notification to IFTTT webhook
            requests.post("https://maker.ifttt.com/trigger/wlan_monitor/with/key/abcdefghijklmnopqrst")
        # write flag
        with open(record, "w") as f:
            f.write(NOTIFIED_FLAG)
else:
    # At least one device to check is still connecting
    if NOTIFIED_FLAG in data:
        # clear flag
        with open(record, "w") as f:
            f.write("")

exit(0)