Just set up a handler to `/wp-admin/index.php` guaranteed to be called multiple times a day for free.

Check EasyCron https://www.easycron.com/

For free on app engine, you can have a cron trigger a local URL which can then invoke a remote URL. I do this every 8 hours (I actually go one further and have that external URL be a Google Cloud function which does a bit of serverless work for me, also at no cost). See here: https://github.com/cretz/badads/tree/master/cron

You could leverage Zapier to do this. Their pricing suggests you can make two-step Zaps for free and that's all you'd need.

Step 1 - schedule Step 2 - outgoing webhook

It takes all of 5 minutes to set things up but we have a pro account so I haven't tried the free tier yet.

Also check out https://cronally.com -- you can schedule HTTP requests or hook directly into SNS.