Cannot manage Memrise subscription via instructions provided by Memrise

The instructions to manage subscription direct me to apple app store. I did not initially pay for Memrise there and cannot fine where I initiated payment. My account has been renewed automatically and about up for another automatic renewal. I would like to select lifetime but have no way of doing anything except hit “cancel” button. Sent email to Memrise but no response except the same instructions in the FAQ that won’t work. Started Memrise on website. Installed free app a few years ago. Became Pro 2 years ago.