Hello Whoilleallues22
Swiss Travel Passes are not valid in Italy, you need tickets from Chiasso to Milan and from Milan to Domodossola. Please note that seats reservations are mandatory in Italy if you take an Eurocity, but not in regional trains. As alternative from Lucerne you can travel with an IC21 and take a regional train RE80 from Lugano to Milan or travel via the old Gotthard panoramic line.
This summer, the line between Milan and Domodossola is closed, you have replacement buses.
Kind regards
Danhiel