troubleshooting – use of ND Variable filter

I am an amateur photographer and I tested a ND 2-400 variable filter-at its maximum 8 2/3 stops-for a low light landscape scenery.My Nikon d610 suggested data were aperture 4.0 ,shutter speed 0.4s,iso 100 without the filter and the exposure calculator app on my mobile gave me a time of shooting of 205 s.However photo was black and remained black even when I exceeded the time of shooting up to 5 minutes and more.
Something went wrong and I need some advice on this.