for InSAR you need SLC products, your coherence image is in slant geometry.
as you said, you calculated Sigma0db based on a GRDH product. This is already pre-processed (squared pixels) so you also need to have the Sigma0db layer in slant geometry. Use the original SLC data to calculate Sigma0.
Please also see here:
You could, as well, geocode both rasters to ground range and then clip to the same extent but this is also tricky.