My messages are sending as texts to an iPhone, but they have internet?

I've texted this person for a long time with no trouble. Even when they didn't have internet, my messages still sent as iMessage, but I recently updated to iOS 11 and now my messages to her are all out of whack. They don't display delivered, I'm not getting her texts and my messages are now sending as text messages even when she's connected to the internet. What's the problem here?

Switch to Android. Never had an issue with Wi-Fi/cellular messaging

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