It is recommended to activate the pass when you're sure you'll board a train that day. This could be at the train station for example. You need connection/wifi to activate it.
Seat reservations are rarely refundable. You can change plans but then you lose them.
If you miss a train due to a delay it's different : you're allowed to board the next available train free of charge (whether in 1st or 2nd class).
Ideally you need to get a proof of delay onboard the delayed train. Act quickly as soon as it's likely you're going to miss the connection. Speak to staff onboard (maybe they can issue a new reservation right away) or at the station.
If the next train has optional/no reservations, simply log the new one on the app and hop on -> it's a lot easier!
Je peux aussi répondre en français si jamais.
No, don't worry, thanks, I understand everything !
Merci pour toutes ces informations !