GT-i9195I not recognized by USB
Samsung devices don't have fastboot, except for Samsung Nexus devices. What you call fastboot is Download mode in which no fastboot commands is recognized. 

I think the computer is detecting the device but not as a fastboot device. On Windows,  go to the Device Manager, you should see Samsung Mobile USB modem which is used as flashing interface. No USB Debugging needed here.

Flashing stock firmware on Samsung devices is easy. Just download the firmware and the flasher (Odin for Windows or Heimdall for Mac).