How can I listen to my mp3 audiobooks on iPhone?

I have a collection of seminars recorded as mp3. In iTunes, I marked them as audiobooks.

I have an Apple music account, so I can access my music library through iCloud, including those I recorded myself or ripped from my CDs.

I cannot access my audiobooks.

I do not see them in the Music app, I do not see them in the Books app -although it has an audiobooks section.

I found some 3rd party apps on the store that play mp3 auidobooks, but I'd prefer using the Books or Music app.

Is there a way to do this? I so, how?

Thank you.