You can manually download the Orbit files and SRTM tiles and place them in the correct folder, so SNAP will not try to download them.
Orbit files: "Error: [NodeId: Apply-Orbit-File] error in opening zip file" and Required format and location for downloaded S1 orbit files
SRTM 3Sec tiles: Unable to get fractional land/water mask
SRTM 1Sec tiles: Process with SRTM 1sec