I wonder if there could be a bluetooth (or connected) high gain antenna attachment for the new iphone they could sell as accessory.
You CAN get high gain antennas for things like your WiFi router, it's my goto when an elderly relative complains about their wifi range.
You can't actually increase power with an (inactive) antenna but you CAN change it's radiation pattern. The higher gain omnidirectional antennas just take a radiation pattern that looks like a sphere (0dB gain) and squashes it into a donut shape. So you get more range laterally around the antenna at the expense of less above and below it