Cancel subscription

Good evening,
Three days ago I bought an iPhone 13 with a monthly subscription. 
I had big issue and delays for having it delivered and this caused me huge problems and I have been totally unsatisfied with tele2 assistance since they couldn't help me at all, they just told me to go to mediaMarkt to buy it. For solving my problems, even if they were regarding tele2 as admitted by the operator, I had to spend 2 hours on the phone with the delivery company and they helped me in finding a solution for receiving the phone.
Due to this troubles and my lost of trust in Tele2 I would like to cancel my subscription with immediate effect and I also would like to have reimbursed the first month I already paid since the sim is still not activated.
Which data do you need for proceeding with the request? 
Thank you