I know this HOWTO is for a different device, but Windows not detecting a phone in fastboot mode is a rather frequent problem and the solution depends on Windows and not so phone model. So the steps described there should help you, too.
About two weeks ago I installed TWRP on Windows on Galaxy S8. With the phone connected, I ran Widows update and some drivers got installed. I even installed USB drivers for phones from -usb-driver.html
Thank you all for your answers, I start to get a better unde