If you have the “standard/regular fare ticket” booked on the 8:15am train and you miss that train you can just get into the 10:15am train (same route), this is correct.
This are the conditions:
- Ticket for a single or return journey
- Duration of validity: valid on selected day
By the way this conditions are also written on your international tickets.
And yes if you have a seat reservation for 8:15 am you must go on the train that goes at 8:15 am so that the seat is valid.
I wish you a nice day
Kind Regards @MelanieD