A custom font can be embedded into the report as a resource, so it travels with the template.

Registering the resource. Load or register the external resource before rendering so the finished report remains portable across environments.
import { Designer, Stimulsoft } from 'stimulsoft-reports-js-react/designer';

function App() {
    var report = new Stimulsoft.Report.StiReport();
    report.loadFile('Reports/CusomFontSimpleList.mrt');

    var fileContent = Stimulsoft.System.IO.File.getFile('Fonts/LongCang-Regular.ttf', true);
    var resource = new Stimulsoft.Report.Dictionary.StiResource(
        'LongCang',
        'LongCang',
        false,
        Stimulsoft.Report.Dictionary.StiResourceType.FontTtf,
        fileContent
    );
    report.dictionary.resources.add(resource);

    var designerOptions = new Stimulsoft.Designer.StiDesignerOptions();
    designerOptions.appearance.fullScreenMode = true;

    return <Designer options={designerOptions} report={report} />;
}

export default App;

The font file is read with File.getFile, wrapped in a StiResource of type FontTtf and added to dictionary.resources. The font is then available inside the designer and in every export.