You can buy a ticket starting from the last stop within the validity of your ticket. This is Chiasso if your train stops there or otherwise just the last stop within Switzerland. You do not need a seat reservation, but it might be a good idea because (only on this route between Arth-Goldau and Bellinzona, specifically the Gotthard Base Tunnel) only seated passengers are transported and it will be holiday season next june, so i would expect increased demand on this train.