Hello Prahoge62
If you bought two tickets in two separate transactions (one ticket until Domodossola and one for Italy), the Agreement on Journey Continuation (AJC) applies, but for me it is not clear if you could use a different route or not. The train operators must be the same.
If the line doesn’t reopen on time, you should ask for advice at the ticket office in Brig, or call the Contact Center as suggested in the previous post.
Kind regards
Danhiel