Just use your old syntax then. It's just important to sync the oData call with the instantiation of your dialog. So in the success handler call sap.ui.xmlfragment().
Hi, I think the problem here is due to the async nature of your oData call. While you instantiate the dialog and open it, the async oData call is triggered to load the data and to set the model. So you have to make sure, that the model data is loaded...