@texasstudet2019-From where I sit, there’s NO chance that Duke will rescind your acceptance. Even so, because you expect to be a CS major and your C was in CS, it would be appropriate for you to write to Duke and explain what you just said here … how the zero on the assignment was the result of your not having WiFi access.
I don’t know if your lack of WiFi was temporary and unusual (e.g., you were on a camping trip or an airplane or visiting your uncle in the middle of nowhere) or if it has been an ongoing issue for you (i.e., no WiFi in your home). But if it’s the latter, you should mention that too.