I am using Pages on an iPhone (iOS 10.3), and I can't add a page break. Apple's support page (https://support.apple.com/kb/PH18098) says to just press the "" key in the format bar above the keyboard, but I don't see it in my iPhone. What I see in my keyboard is just this:
As you can see, there's no format bar above the keyboard anywhere. Maybe it does show up on an iPad, but as I said, I'm on an iPhone.