I just tried the operation you're describing. I have an iPhone 7, iOS 11.1. I do not use iCloud Photo Library, but I do use Photo Stream. The video appears in my Camera Roll and I am able to edit and trim it, just as you desire.
So I can't imagine what's going on. Perhaps you should try resetting your iPhone. Or perhaps you're filling up your iPhone's storage (check Settings > General > iPhone Storage and be certain you have at least 6½ - 7 GB remaining).