A:AnswerIt works with my D750. Should work if the D800 has front and rear Infra Red detectors. It is IR transmitter and must be "line of sight" to the camera.
A:AnswerThe remote appears to be a knockoff of the Nikon ML-L3 infrared remote. I looked at the Nikon product page for that remote and it's not showing the D850 as supported by the name brand IR remote, so I don't suspect the Insignia remote will work. I won't post the Nikon URL here, but search the Nikon site for ML-L3 to see the product page and supported product list.
A:AnswerWe are not able to test this shutter control with all Nikon cameras so we cannot guarantee that it will work with your specific model. If this model of camera has an infrared sensor it should work though. Best Buy does have a 15 day return policy if it does not work as intended.