Remove the slide. Then remove the left side grip panel. Now using a small screwdriver or similar tool push the stainless steel looking flat bar forward. This is below the slide release, inside the gun. Just ride the hammer forward with your thumb. From here reinsert the mag and squeeze the trigger to see if the connector is operational. If so, try the decocker again to see if it is functional. Or as others have suggested, just send it back to Sig for them to fix