I sent the rent contract cancellation letter via “Superpost”, landlord not confirming, should I be concerned?

I sent the rent cancellation letter via Superpost. Superpost is a service (https://www.superpost.com/en) that allows you to upload a letter online, then they print it out, and send it by post.

They say "If the delivery fails, we will notify you. If you don’t receive a notification, you can assume that your letter has been successfully delivered." I did not receive any notification.

I sent the letter on the 26th of June. I have been asking the person managing my flat at the Real Estate agency (who usually was very responsive) to tell me whether they received the letter. He has not been replying to me for two weeks now. I have called the real estate agency twice now, and they tell me that the person responsible for my flat is not available and should leave a note. But I am pretty sure he does not want to talk with me. I have no idea what is going on.

Now, am I safe legally? Or is there a risk I have to pay also the rent for October?

If I am not safe, should I send a registered letter (Einschreibebrief) now to be very sure? In this case, I would love the money for the October rent for sure though.