While I live in Colorado, our son lives in Brooklyn, so we go up to visit a lot. I've never found AT&T service to be very good in NYC, NY, Vermont, NH, NJ, and Connecticut. I've worn out the "Mark the Spot" app used to tell AT&T about crappy service...not sure it's doing any good...as, it's still crappy.
For instance, we went to the Vermont County Store, which is in a small town in southern Vermont. My iPhone 6s Plus had No Service, but other people with Verizon, T-Mobile, and Sprint had no issues with either phone or data.
I hope AT&T decides to do a better job on coverage, or, I'm going to switch to Verizon.