Boy, those 5G phones really are strong. If you try to explain it, the concept of time travel as a cinematic device is pretty preposterous. It’s like a magic eraser that lets the screenwriter fix any plot problems, because, you know, time travel. And yet, when done properly, it cashes in your willing suspension of disbelief and pays you back tenfold. “Don’t Let Go” (2019) asks you to believe that the magic of cellular technology can allow a cop (David Oyelowo) and his niece to try to solve two murders, two weeks apart. It works. Just promise to stay connected.