Moodebeeth19 If you have a Saver Day Pass, you can use every other train in Switzerland until Chiasso (the border station with Italy).
After Chiasso, it is more complicated. You can also use every train, but you must make an online check-in. You have two tickets for every passenger: the first one between Spiez and Chiasso and a second one between Chiasso and Milan.
To make the check-in, I suggest to install the mobile app of Trenitalia on your smartphone. The second ticket has a numeric 10-digit code. You can import the ticket in the app with this code and your email-address. If you are on time (15:02 from Lugano, 15:31 from Chiasso), simply make the check-in when leaving Lugano. If you miss the connection in Lucerne and you are late (probably 16:02 from Lugano, 16:31 from Chiasso), select the new timetable from Chiasso to Milan in Trenitalia app and make the check-in.