Along much of the Ukraine-Russia front line, commercial cellphone towers providing voice and internet service have been knocked out by power outages, Russian shells and jamming. But Elon Musk’s Starlink service, which provides internet connectivity using a swarm of satellites, is up and running here. Mr. Musk has sent thousands of kits into Ukraine, turning the country into a high-profile testing ground for the service, which he is trying to sell in other countries around the world.