Contract cancellation

I've moved from the EU to Latin America. O2 doesn't offer services here, so I want to cancel my contract. I've sent two letters to terminate it. The first was on September 11th. I received an email saying it was under review, and in October I received another bill even more expensive cause of a roaming I didn’t use cause as fast as I landed I turned off the data. I sent another letter and again received a notification that it was under review, but no confirmation. I'd like to know who I can contact remotely, preferably in English. I've sent emails and haven't received a response. The last time I contacted them, they said they couldn't find any cancellation letter. I no longer have my SIM card, so I can't make calls without incurring exorbitant costs, which is why I'm communicating via email. When I signed up for the prepaid SIM, I was told I could cancel at any time. I always get the option to end my contract in my O2 page but I can't find where.