I would advise against trying to use the built-in cronjobs while using heroku, much like the reason why we don’t support the CTB or SQLite on Heroku: Troubleshooting - Strapi Developer Documentation
Heroku instances have a limited lifetime, typically only a few hours before they go to “sleep”. At the point of sleeping the instance is effectively killed and no longer running, meaning cronjobs aren’t running. When Heroku detects new traffic, it spins up a new instance, pulls the code from Git, and cold boots the project.
Meaning if your instance is “sleeping” when your cronjob is supposed to run, it won’t. Attempting to force the instance from going to sleep will not be possible as Heroku specifically states they will forcefully kill the instance: Free Dyno Hours | Heroku Dev Center