"Connection Error Detected" on iPhone app

When I try to download an ebook (.acsm file type?) on the iPhone an error pops up saying:
"Digital Editions could not connect to the fulfillment server. Please make sure you are connected to the internet"

I am connected to the internet and I have signed in to the app with my username and password, which is the same as what I use for the version of ADE that I downloaded onto my Mac laptop (which has no issues downloading books).

What do I need to do to be able to read these books on my iPhone?!