Service Desk notification problems

Hi
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:
https://docs.gitlab.com/ee/user/project/service_desk.html#as-an-end-user-issue-creator

I tryied few things:

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

I am a free user of the online gitlab.com version.

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