If you’ve download the ASTER tiles, placed them in the right folder and they follow the naming convention ASTGTM2_N00E111.zip then you don’t need to use them as an external dem file. You should select ASTER from the DEM pull down menu.
It’s possible the file naming is not correct and SNAP is not recognizing it. In which case the software will not know the difference than if a tile doesn’t exists.
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