this is a quite outdated solution and refers to the mt_prep_gamma_snap script which is no longer used.
So if the StaMPS export created the lat lon files and you ran mt_prep_snap successfully, I don’t think it will help to re-create the lat/lon bands manually.
But if you want to give it a try, you can replace the files in the DEM folder as suggested here: How to prepare Sentinel-1 images stack for PSI/SBAS in SNAP (step 9).
I rather suspect that snaphu might not be running correctly. What happens when you type snaphu in the command shell (not in matlab)?