Appreciate the follow up @Jiaszzz_ASUS
but I've successfully used the latest version of the app and it's worked fine, as such the app is not the issue, rather than the 5z doesn't have accessible drivers when it is in fastboot mode, therefore it cannot be connected to via Windows 10. Without the correct IDs, it is not possible to update the device via Fastboot. If it is possible to get the correct VID & PID for a phone plugged in via fastboot mode, drivers, or both, it'd be appreciated. Otherwise I can try to get the VID & PID from the Zenfone 3 in fastboot mode, and hope they work for each other.
In the USB support files on your site titled "ASUS_Android_USB_drivers_for_Windows", inside the zip there is the file "android_winusb.inf", inside which there are records which tell Windows 10 which driver to use, for example:
%CompositeAdbInterface% = USB_Install, USB\VID_0B05&PID_4D05&MI_01
%SingleAdbInterface% = USB_Install, USB\VID_0B05&PID_4DAE
%SingleBootLoaderInterface% = USB_Install, USB\VID_0B05&PID_4DAF
None of these line up to my device. Because my PC cannot determine what the device is (error = Unknown USB Device (Device Descriptor Request Failed)) it allocates a generic VID & PID (USB\VID_0000&PID_0002), so I can't find out what driver to apply. I need to know the correct one so I can get it to recognise.