good job on creating the interferogram!
Is the external DEM referenced to WGS84 and stored as a Geotiff?
I don’t see the common area between your InSAR result and the DEM, to be honest.
You can include the external DEM in the output product by selecting it in the Terrain Correction model. Then you can look at the DEM and the InSAR DEM at the same time, for example by a scatter plot (example here) and also calculate a differene image.