I don’t have the 18i8 ( I have an old Presonus Firestudio, similar in’s and outs I believe)
I run my guitar to input of KPA and Spdif out of KPA to Spdif in of Presonus. With software supplied you should be able to see levels and what channel(s) are active (in my case 9&10).
In GarageBand I open a track and when choosing the track you can specify which channel(s) as the input. Once you see signal on that track adjust if needed by the input meter.
Not sure if that helps but that’s what works for me