SIM contract cancellation due to leaving Germany

Hello,

I signed the sim-card contract with O2 for 24 months on the 05.11.2022. I am from Ukraine and I am going back there on 20.02.2023 (I have the tickets), so I already have my de-registration paper. I read on the forum here that I’m supposed to send this de-registration form and a proof that I’ll actually stay abroad (paper from the landlord or employer) to Telefonica address.
The problems are: first of, I do not rent an apartment, I own one, so there is no landlord I can ask to write anything. Secondly, I work for myself and do not have an employer I work for. So, what am I supposed to do now? What documents can be sent as a proof, apart from de-registration paper (maybe my Ukrainian passport would work)? And on what address should I send all documents? Will there be any fees? If possible, please provide the example of how this letter should be written, because German is not my first language.
Thank you beforehand!

Moreover, when I went to the O2 shop (around 13.11.2022) I signed my sim contract at, for some reason they told me that it is not possible to cancel the sim contract within 14 days, but only if I am leaving the country (at that time I was not planning on leaving Germany for good due to war escalation). Once again, I read on the forum here that it is actually possible to cancel the contract within 2 weeks. Very strange misinformation. (ALREADY CLARIFIED, NO LONGER RELEVANT)

With respect,
Zoya