(Getty, Photo Illustration by The Real Deal) When fast-fashion chain Mango agreed to sublease Ralph Lauren’s 28,000-square-foot Fifth Avenue store last November, the deal seemed like a win-win for both retailers. Mango would be able to explore the idea of having a store on one of the city’s most iconic shopping corridors, at a fraction of the cost. Ralph Lauren, meanwhile, was struggling due to the pandemic, and would have a chunk of its rent paid off. But the deal hasn’t worked out quite as either hoped — and shows some of the challenges inherent in retail subleasing, which experts believe could become more popular as tenants struggle to make rent and landlords look for options.