my thesis was about permafrost degredation in greenland using S1 imagery and experienced the same issue you have.
I have used both TanDEM-X 90m as external dem and GIMP(Greenland ice mapping project) DEM with 30m resolution. depending on your situation you can choose betweeen these 2. i would highly recommend to not use Arctic-DEM because it has bad height resolution for Insar purpose.
You can download these dems online and supply them to SNAP or SNAP2StamPS using the external dem function as a GTIFF in unprojected wgs 1984.
you can check my github for my documentation on how to do this in SNAP2StaMPS.