I will be traveling from Zermatt to Milano Centrale with a big party - 8 people - on June 15th. We want to take the 11:13 train via Visp. We want to make sure that each group of 4 are sitting together on the train from Visp to Milano Centrale(I know there is no reservation from Zermatt to Milano). The site says that seat reservation is available, but I’m unable to select a seat. How can I make sure that our seats are together?