Thanks for your explanation and patience with me ??
What the plugin does is automatically trigger an update of a ‘synced field’ each time the matched ‘primary field’ changes.
The plugin is not designed to manually sync, it will always sync if the primary field changes. The only reason there is a ‘manual sync’ button is to enforce the update of the synced field if a cron job on the WordPress site has not fired yet. WordPress uses visitors to trigger cron jobs and if there are no visitors yet (for example because the site is in development or just has low traffic) it means no cron jobs are triggered. Once the cron job finally runs, the synced field gets updated after all.
I’m afraid syncing just once a month manually is not going to work with this plugin. If there is nothing to sync (the primary field hasn’t changed) the manual sync does nothing, and if there is something to sync (the primary field HAS changed) the manual sync button only updates if the cron job hasn’t fired yet, (and if it has it does nothing but doesn’t harm either).
Let me know if I missed your point or didn’t explain properly.