To set up cron for re-indexation, refer to the following article: Configure and run cron.
After that, in the Magento admin panel, navigate to Stores > Configuration > Searchanise > General, and select “Periodically via cron” in the “Sync mode” settings.
The cron start time can be modified in the app/code/Searchanise/SearchAutocomplete/etc/crontab.xml file.
