sorry, I didn’t see it before: flat-earth phase estimates the ellipsoid of the earth’s surface and how it impacts the phase in your area.
If you want to perform differential interferometry (DInSAR) for the mapping of displacement or subsidence, you need to check “subtract topographic phase” as well, because then all topographically induced patterns (as we might see here) will be reduced. Use SRTM 1Sec HGT AutoDownload.
This will leave you with an interferogram that only contains the phase difference between both aquisitions (and no longer the contribution of elevation changes).