Hi Marco,
You can refer to topic https://jazz.net/forum/questions/109256/how-to-set-up-rqm-reviewapproval-cycles-and-expect-email-notifications which has detailed explanation to set the email preference.
In general, you need to edit the Mail Configuration from the user's profile, enable "Email me for approval changes that affect me".

Please also note, if the approver is not belonged to the project area or team area of the work item, the approver will not get email notification as he could not access the project to see the work item and take action.