when all images are from the same track, you get the displacement along the line-of-sight (LOS) between the earth and the sensor.
If you want to have vertical displacement, you need images from both ascending and descending tracks. There are lots of approaches to achieve this, some are strightforward, others are more complex. At the moment, none of them is implemented in SNAP.
- Toward mapping surface deformation in three dimensions using InSAR
- Inferring three-dimensional surface displacement field by combining SAR interferometric phase and amplitude information of ascending and descending orbits
These things are also discussed in this topic: Difference between vertical and LOS displacement