You cancel in the app store, not in the app. That is where the billing runs, and it is the only place it can be ended.
Android (Google Play): open the Play Store → tap your profile picture → Payments & subscriptions → Subscriptions → Citily → Cancel subscription.
iPhone (App Store): open Settings → tap your name → Subscriptions → Citily → Cancel Subscription.
After cancelling, your access stays until the end of the month you have already paid for. You lose nothing by cancelling early, and nothing is refunded pro rata. After that your account remains, and only the content is locked again.