I think this is W.M. Aker's Westside
"Westside" is set in an alternate 1920s in which a 13-mile-long fence runs down Broadway to split Manhattan between the sunny and prosperous East Side and a West Side that's historically held the city's deplorables, what the narrator, Gilda Carr, 27 years old and a private eye, calls the feckless, greedy, criminal and mad.
Gilda is a West Sider. And like her father, a gang leader who became a police captain, she tries to solve what she calls tiny mysteries, like a missing brooch or a piece of millinery. But Gilda Carr finds that the case of a missing thin white glove will throw open both sides of the city and pull her into the riddle of the city itself and into a looming turf war between bootleggers and crime lords that rekindles the mystery of the disappearance of her father.
From a Goodreads review:
Speaking of vanishing, in this magical reimagining of the Manhattan of 1921, considerable bits of the island have been doing just that. Odd objects, coffee pots, stairway railings, entire buildings are being swallowed up by something. This is not totally new. Akers notes an apocryphal 1628 letter from early arrival Peter Minuit about the oddity of the west side of this newly colonized island. (Our homes shift on their foundations…Our wood comes loose from its joints, and my dreams are plagued by visions of pestilence, stigmata, and the armies of hell.) Things tend to degrade faster, rust races instead of creeps. Machines cease working. Guns fail, automobiles sputter. The trees do pretty well, though, growing tall and fast. Streets become streams instead of the other way around. Occasional waterfalls form and descend from rooftops. It is where Gilda lives. In a brownstone facing Washington Square Park (mom came from money).
And then there is the increasing vanishing of humanity. Enough so that when over three thousand people went pffft! on the Westside in 1914, thirteen miles of fence was erected down Broadway to separate the Westside from the rest of Manhattan. Not her problem. She can get back and forth through the security gates readily enough.
Gilda is engaged by one Edith Copeland. It seems Mrs Copeland had mislaid a glove, one of a pair her oft-absent husband had given her as a gift. She would like the glove found and returned, as she does not want to face awkward questions about its absence. But in this version of New York, tiny mysteries have a way of leading to very large questions, and Gilda’s gumshoeing leads her to a very, very dark side of the city.
Found with a search for fantasy novel "new york city" iron fence (after several false starts)