Depends on how much you want to set up. For my purposes, I just check for connectivity every minute, and record true or false as a new row in a sqlite database if there is connectivity.

This is what I use on my raspberry pi,

#!/usr/bin/env python3
from datetime import datetime
import sqlite3
import socket
from pathlib import Path

try:
    host = socket.gethostbyname("one.one.one.one")
    s = socket.create_connection((host, 80), 2)
    s.close()
    connected = True
except:
    connected = False
timestamp = datetime.now().isoformat()

db_file = Path(__file__).resolve().parent / 'Database.sqlite3'
conn = sqlite3.connect(db_file)
curs = conn.cursor()
curs.execute('''CREATE TABLE IF NOT EXISTS checks (id INTEGER PRIMARY KEY AUTOINCREMENT, timestamp TEXT, connected INTEGER)>
curs.execute('''INSERT INTO checks (timestamp, connected) VALUES (?, ?);''', (timestamp, 1 if connected else 0))
conn.commit()
conn.close()

and I just have a crontab entry * * * * * ~/connectivity_check/check.py >/dev/null 2>&1 to run it every minute.

Then I just check for recent disconnects via:

$ sqlite3 ./connectivity_check/Database.sqlite3 'select count(*) from checks where connected = 0 order by timestamp desc;'

Obviously, it's not as full-featured as something with configurable options or a web UI etc, but for my purposes, it does exactly what I need with absolutely zero overhead.