I'm sorry, but what do you mean by 'default' calendar? Samsung devices only ship with one calendar app: S Planner (might be renamed by your carrier, but it's still S Planner).
You can have multiple calendar apps installed and running in your phone, they won't 'conflict' with each other. The reason your events might not be showing up might be that you're saving your events in a calendar that is not being 'read' by the app you're referring to. For example, if you create an event in your Device, that is most likely to show up in any app. But if you create an event in your Samsung account or your Google account, unless the app you're using can log into those accounts, those events will not show up in that app.