I am going to assume you have already got your NAS working, with a working install of WatchTower.
Once that is working you want to edit the environment variables.
Add WATCHTOWER_NOTIFICATIONS = shoutrrr
Add WATCHTOWER_NOTIFICATION_URL = telegram://[TOKEN]@telegram/?channels=[CHANNEL-ID]
*edit token and channel id to match your own channel.
To get your token create a Telegram Bot by messaging @BotFather.
Your token will be a mixture of random latters and numbersimilar to this format 1111111111:AAAAAAAAAAAAAAAAAAAAAAAAAAAAA
In your Telegram channel send a message to your bot such as
Once this is done, you can forward a message in the channel to @getidsbot
and get your ID
Back over to the Synology NAS
Add the following to the WATCHTOWER_NOTIFICATION_URL = telegram://
So it should look similar to the below
Start the watchtower docker and you should get an alert