Despite I selected 120 minutes for SMS reminder option, the SMS is sent at the time of the appointments. Could you solve that ? Thank you.
can you please tell have you connected WordPress cron to system scheduler? If that is not the case then cron execution depends on site visits. So for example SMS will be send when first customer visit site if there is less then 120 minutes to appointment. So it can happen that nobody visit the site for like an hour etc.