First of all, the Advanced Roadmaps hierarchy is now supported in both versions, Cloud and Data Center/Server.
Taking into account the nature of Parent field in Cloud Jira versus Parent Link field in Data Center/Server Jira, there are some differences when it comes to Advanced Roadmaps hierarchy support by Gantt Suite depending on the app version.
Not sure what Jira version you’ve been using, Cloud or Server/Data Center?
The easiest way to see if you are on Jira Cloud or Server/Data Center is to look at the menu, if the main menu for Jira has category Apps you are on Jira Cloud. If the apps are directly at the main menu you are on Jira Server/Data Center.