I noticed that you have opened a Support ticket with the same question. We have replied to this ticket. For the benefit of our community, I will include the reply also in this forum post:
If the custom REST APIs is providing data for the report, our recommendation is to set up a WebServiceDataSource to access the REST APIs data.
Generally, the Angular Telerik Report viewer requires its own REST API to access the reporting engine. The service setup requires the implementation of the ReportsControllerBase.
If additional assistance is needed please elaborate further on the case and in case of service errors please send us the Fiddler SAZ log.
You may also test and go through our Angular demo that comes with the installation of Telerik Reporting. By default, it can be found in (Telerik Reporting installation folder)\Examples\Angular\CLI (e.g. C:\Program Files (x86)\Progress\Telerik Reporting R3 2019\Examples\Angular\CLI). The example utilizes the REST Service from our online demos (https://demos.telerik.com/reporting/api/reports/).
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