To find your active subscription on an iOS device, tap into the 3-bar menu on the left-hand side at the top of the page. Tap "My Books". There, you'll see your subscription(s) and Drafts. Tap into the Monthly Mini or Monthbook subscription labelled ACTIVE. You can now make and edit your Monthbooks there.

Here's a video how-to to find this as well:

iOS: How to locate your active subscription