Apple Inc. has been secretly working with SpaceX and T-Mobile US Inc. to add support for the Starlink network in its latest iPhone software, providing an alternative to the company’s in-house satellite-communication service. The companies have been testing iPhones with the Starlink service from Elon Musk’s Space Exploration Technologies Corp., according to people with knowledge of the matter. In an under-the-radar move, the smartphone’s latest software update — released Monday — now supports the...