The first condition for the reminder emails is: in the AttestationHelper table, for every decider there is saved an entry for when this person will be next informed (reminded).
Confirm by checking AttestationHelper.NextReminder. The reminder time is saved in UTC there.
If there are values which are in the past the reminder should be fired. The schedule "Attestation check" is responsible for this.
Check the settings of this schedule in Designer: Base Data | Schedules. Possibly it has been deactivated or the settings were not changed. By default this schedule runs daily at 6 PM UTC.
When this schedule runs AND there are any outstanding reminders AND there is a mail template defined in the workflow the process "Created by QBMDBQueueProcess: fire event Remind for object type AttestationHelper" will appear in the Job Queue. And that process should trigger the process to send the mail.