Complaint - I cancelled the contract, but still was charged!

I was an Erasmus student in Heidelberg for 6 months, from 1 September 2015 to 15 February 2016. On 7 September 2015, I bought two internet usb sticks (and started two internet provision contracts) from the 02 shop at Hauptstr. 118. I told the employee I wanted them for 6 months and he asked me to come again in February 2016 to cancel the contracts before I leave. Indeed, on 8 February 2016 I did so, I returned the sticks and gave a paper from the Studierendenwerk, proving that I leave Heidelberg. On 30 March 2016, the employee sent me an e mail confirming that he cancelled the contracts. Indeed, from 30 March 2016 on, the automatic payments from my bank account stopped (only O2 could do this) and I could no more login to MyAccount (it was deactivated).
A few days ago, after so many years, a letter came in Greece from a debt collection agency, stating that the contracts were cancelled by O2 on August 2016, that I owe 712.82 euros to O2 and that they were sending letters to my student residence in Heidelberg after my depart!!! Half of the money concern a mobile device, which I have never bought!!!
As I am severe hard of hearing (schwerhörig), it’s impossible for me to communicate through telephone. On 30 August 2018, a German speaking friend has called both O2 and the debt collection agency. O2 said that they never received anything from the shop (O2 help desk : “yes we see something changed in February 2016, but the contracts were not cancelled then”). They don’t accept the employee’s e-mail (debt collection agency: “If you don’t have an official paper of contracts termination, you have to pay”).
The shop employee confirms that he has sent everything to O2 on February 2016. On 4 September 2018, a friend visited the shop 02 shop at Hauptstr. 118, the shop employee still works there and confirms all the above. He remembered me because I have hearing aids (he said spontaneously that I gave him the paper from the Studierendenwerk). When asked to call O2 and the debt collection agency, he said that they can call him.
I have contacted both O2 Facebook and English O2 community by posts and the debt collection agency by e-mail, complaining about all the above. I have not received any answer yet.

As I am not a debtor, I would like answers to the following questions:
- As a consumer I went to the O2 shop in Heidelberg and asked for my contracts’ termination. I trusted the employee (who assured me both orally and via e-mail that everything was ok), shouldn’t I? If he doesn’t know the O2 procedures, why is he still there after 2.5 years?
- How does it happen that half of the money I “owe” concern a mobile device, which I have never bought?
- During my stay in Heidelberg, I paid for 6 months via automatic payments from my bank account. As only O2 could stop them, how did the payments stop on March 2018, if my contracts were not terminated?
- During my stay in Heidelberg, I could login for 6 months in O2 myAccount. Why O2 has deactivated myAccount on March 2018 (I could no more login), if my contracts were not terminated?