In general, the HTML5 Report Viewer is a client-side widget served by a running Reporting REST service, which is dedicated to work with the viewer. It is the service that manages the report state and resources. More details how the viewer and the service work, and how to set them in a project are available in this forum thread.
When you want to display a TRDX file, you need to use a ReportFileResolver, where you need to specify the folder on the server that contains the TRDX files.
You can also test our local demos installed by default under C:\Program Files (x86)\Telerik\Reporting <VERSION>\Examples.
projects use the folder with TRDX files installed with the Standalone Report Designer.
Also, you can use Fiddler and record the requests from the viewer to the service in a log file. In Fiddler - Inspectors - Request/Response - Raw tabs you can see the request and the response content, and any errors that occur.
If you need further help, feel free to open a support ticket and send us the log file created by Fiddler and a demo project which illustrates your settings.
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