This is what I did with Heroku and will continue to do if I am trying out fly.io or render (1 dollar for cronjob). I know you could get free cronjob on Heroku if you provide them your credit card info.

So, here is what I did. Added a simple API layer in front of my script. The API layer takes in a secret message to a specific endpoint that initiates a background process. Initially, I used my raspberry pi to periodically make the API requests. But I plan to try out: https://cron-job.org/en/