DANGER! This precodure worked for me. I can't guarantee that this also works for you. You might brick your device.
I wasted the last 2 days but I found a way to update the firmware of my Netstream 4C from version 1.1.0-365r1 to version 18.104.22.1682r1V1. The FAQ and the articles on the Geniatech site are a lie. You can do it with MacOS, Windows or Linux but not with your smartphone (or maybe you can if you can find a super old version of the EyeTV netstream phone app but I don't