Currently, when the detail is added to the portal, the portal users cannot add the records. This is default application logic. We already had multiple requests from the customers to make this option possible and this idea has been accepted. It will be possible to add the records on the detail on portal in the nearest application releases.
As for editing existing records, the logic of the portal affects overall detail functionality. It affects the existing records as well. Basically it is not quite possible to use details on portal since they do not have the corresponding records in the SysModuleEntity and SysModuleEntityInPortal to work properly on portal.
The data available for portal users is limited by the [ List of objects available for portal users ] lookup. Only the objects included in the lookup are accessible via the portal UI.
In addition, you should configure your object permissions according to your detail object.
