Oh sorry, my bad… 
@Corentin and @Conatus are trying to return to FP OS with GMS. They did try with method 1, but it didn’t work. So I told them to try method 2 ! But for Windows commands, I have to guess or the make researches, as I don’t know the Windows Command Prompt at all (I am on Linux).
As you seem to be a Windows user and to know cmd, I thought you could help them to navigate through the files with this tool.
@Corentin : you need to navigate to the folder in which the ZIP file is located, for example “Downloads”, and then type the command. Did you download the drivers for your Windows installation ? There you see that you’ve got two links to follow depending on your installation : https://forum.fairphone.com/t/pencil2-adb-on-the-fairphone-2-windows-driver/11529/15