Read this thread and are having a similar problem although not exactly as it is mentioned. Clients are detecting when not on VPN that they are internet clients and checking into the CMG and reporting back. However the software center is not available to install device targeted apps. We have removed the deprecated application catalog website and application catalog web service roles from the site system (client settings have been updated) but it seems that the client on the internet is still trying to reach the Site server by its internal DNS name which is not resolvable externally. It still lists the following "GetCategoryValuesAsync: There was no endpoint listening at http://Internal/CMApplicationCatalog/applicationviewservice.asmx that could accept the message. This is often caused by an incorrect address or SOAP action. See InnerException, if present, for more details.. Unable to fetch user categories, no endpoint found." as if it is trying to connect directly to the app catalog webservice role.
Followed by "GetCategoryValuesAsync: Object reference not set to an instance of an object.. Unable to fetch user categories, application catalog role is probably not installed. (Microsoft.SoftwareCenter.Client.ViewModels.SoftwareListViewModel+<UpdateCategoriesAsync>d__126 at MoveNext)"
Any ideas?