Apologies for the delayed response. The options for ad monetization in apps are incredibly varied and new companies and products are popping up all the time.
Your options will depend on the structure of your app, how you engage your audience, content makeup target demographic to name but a few.
Are you looking for programmatic demand? Direct sales? What's more important to you, fill or CPM?
Sorry to bombard you with questions, but mobile advertising is a very crowded and complex field and the right choices can affect your revenue and audience retention significantly.