VlnLLL Sorry for the delay with my answer.
If you travel this summer, until September 8th 2024, the line is closed. You have direct replacement buses from Domodossola to Milan.
You can book these buses on trenitalia.com or with the Trenitalia app. Please note that other regional trains/buses are available but you have to change at least once between Domodossola and Milan. You can use your Swiss Travel Pass until Domodossola.