Service Desk notification problems

I’m testing service desk with customers.

My customer open the Ticket and receive the thank you notification:

Thank you for your support request! We are tracking your request as ticket #12, and will respond as soon as we can.

BUT doesn’t receive any email notification when I comment the issue or I close it.

this is what I am expecting as customer:

I tryied few things:

  1. manually activate notification on the generated issue + manually change the issue to Not Confidential
  2. customize and message template (the thank_you message changed)

I am a free user of the online version.

How can I activate comments notification by email for my customers?