I think it's best to cancel your tickets you bought from interrail and ask for a refund. But check the conditions carefully.

I see you live in Germany. 

For new tickets, visit a station in Germany or call them to buy your tickets via DB BAHN.

Thanks for replying! I already thought about that too, because conditions say that I‘m able to receive a refund, but I‘m really afraid I won‘t get my money back. Do you know how long it takes to get the money back after cancelling the reservation? And also: Sadly I‘m not able to buy them via DB Bahn, because they say it’s not possible. Would you rather recommend me to try buying reservations via interrail again or to use trains without a need to buy a reservation?

It is also possible to get these reservations online via ÖBB with the one way ticket option and as discount Interrail.

Normaly I suggedt to go to a Ticket Office in Italy because they can change it, but you are starting in Switzerland. From Zurich to Chiasso it no problem, you take a seat in 2nd class, reservations are optional in Switzerland. You can ask the swiss conductor if he can help (I don't think), you could also ask in Chiasso the Italian conductor that enters there in the train. If the says no, than you can exit the train and take one of the frequent regional trains (no reservation needed) to Milano or pay 20€ onboard for a new Reservation.

@Marion Hey, it is a year that this problem is cousing chaos for travellers. Can you please check with the reservation team if they can solve it? Or a warning so people try via ÖBB instead.

I don't recommend buying tickets through interrail.

Who told you at DB BAHN that they can't reserve seats for you, for these trips?

And no, I have no idea how long it will take before you get your money back. It is certainly mentioned somewhere on the interrail website, but I can't find it right away.

Attention, for seat reservation refunds, it is the conditions of the train companies that apply

These exchanges  of 1st/2nd are tipical and very often repotred (both ways in fact) for Italy-this is just 1 of the many pitfalls of the current system of Inter/Eurail-but they wont bulge in f.e. giving people the passcover nr to do it quickly themselves. There was a report from service a week or so ago that they seem to have solved it-but it seems not-or maybe partly.

Thank you all for replying, you‘ve been very helpful. I now tried to book reservations via ÖBB and DB Bahn websites, but both sites tell me when I type in the train connections that it is impossible to book reservations on these connections. So, I guess my only option is to wait for the costumer service to reply, if they ever would? I would also try to get refund because I checked the italian railway company conditions and they said it is possible to get a refund, but (as non-reliable as interrail seems) I‘m afraid I won‘t get any money back if I cancel the reservations on the interrail website and be left without the reservations in Italy and need to take the regional trains. :(

Oh, I just realised I forget to tick the one way option on ÖBB website. Thanks, Angelo, this seems to work! :))

the only thing left right now is how I will get my money back from interrail?

@Angelo Do these reservations bought via the ÖBB website get accepted by the train conductors in Italy?

I never travelled with it, but people are not complaining about problems with it. 

Thank you for all your help! Now, as I would like to save money, what would you guys advise me to do? Wait for the costumer service to reply and change the tickets (even though I don’t know how long this will take and if they will ever reply, I‘m afraid seats in 2nd class are sold out until then?) or book seat reservations now via ÖBB?

I also forgot to mention that I only have a mobile pass, not a paper pass. Maybe this will make any difference for the process.

Nope, no difference.

Customer service will respond eventually. Just make sure you get a full refund since this was an issuing error.

@rvdborgt okay, thanks! Am I getting a Full refund through the customer service or do I have to do something else?

Only through customer service. If you cancel reservations yourself from the Reservations Overview (if a refund is possible at all - that depends on the train), then it's a cancellation of your own choice and they'll probably deduct an admin fee.

I would wait for the customer service. Because like I said, you can avoid on this route the the trains that need reservations from Chiasso to Ventimiglia. If you have time you can take from Chiasso only regional trains (RE or RV). Also there are IC from Milan to Ventimiglia they cost only 3€ in Italy or booking via ÖBB.