The hottest new accessory in the Hamptons this summer isn’t from Hermes or Loro Piana. It’s a clunky cell phone signal booster that exasperated Hamptonites are affixing to their cars to improve their reception in the notorious dead zone. “There is better cell service in Bangladesh than here,” one Sag Harbor resident complained to The Post. The devices, made by companies such as weBoost, Wilson Amplifiers and Cel-Fi, sell for around $500. They have long been popular with truck drivers on long-haul cross country trips or adventurers traversing the Rocky Mountains. But now they’re in vogue with a more posh...