@Gijs Thanks a lot! I was now able to implement it with Node-RED querying the TTN Gateway API as you suggested, and sending a Telegram message whenever the status of the gateway changes from on-line to off-line or vice versa. The link you provided gave me the script needed in Node-RED. And the following YouTube video showed me how to set up a Telegram bot: [https://www.youtube.com/watch?v=HsM4CbMGWmU&ab_channel=ElectricalEngineering](link url).
It took me a while to also realize that you need to specify your personal chat ID in the Telegram Payload node in Node-RED. Once I did that, it worked like a charm.