If iTunes 12.8 on macOS High Sierra isn’t seeing iOS devices, you may get a prompt to install software to see the iOS device, and then the installation may seem to take a while. For example, you may see this for a long time (over 20 minutes):

And even this might stay for a few minutes:

Once that’s installed, it should work. If you don’t see that prompt on one machine but do see it on another, you can grab the installer from /Library/Updates/041-14491 and install that manually.