The construction work will only affect the route between Milan and Domodossola from August, 21st to September, 10th.
To travel from Milan to Domodossola, replacement buses and/or regional trains are provided. Trains from Switzerland to Domodossola and from Domodossola to Switzerland are not subject to changes.
The EuroCity 54 (16:48) from Domodossola to Basel SBB is guaranteed, as is the InterRegio 3030 (17:48) to Brig. You can therefore trust the online timetable, which is up to date.
I wish you an amazing journey in Switzerland.