Once you bust through that 15-20mins you are home free as a reader. Take your book, leave your phone, and read outside. That way if you want to abandon your book to find some little dopamine hit to stay your ADD, you'd have to go all the way back inside.

Even just another room or nook works, or the public library, so long as all you do there is read. You'll begin to associate that spot with reading and have an easier time getting into the reading mindset.