Annual subscription of a magazine stopped working

I have bought an annual digital subscription of a magazine in May 2018. The new owner of the magazine introduced a new app for iPad and since then I have been unable to login my account. Have had several contacts with their customer service, they have said that they are not able to find my account. No possibility to ask for a new password or anything as my email address is not found in their database anymore. I have provided them with evidence from iTunes that I have made the subcscription, still no reaction from their side. If they remain unresponsive to actually solving my issue, is there a way to approcah them through Apple?