Not sure about the US, but here in Europe you need a licence for any radio equipment that emits more than half a watt. With less than that, you're not going to cover that distance for sure. And even if you don't need a license, it's unlikely that mobile devices are going to form a reliable connection over such distance in a large city. Usually to get those types of coverage, relays on the tall buildings in between are required.

Most logical and reliable option to me seems some kind of Push-to-talk over Cellular (POC) solution. You have mobile radios (walkie talkies) that have them but they would require a subscription for their cellular connection. There are also phone apps that offer this functionality, and I suppose you already have a cellular subscription for your phone. I'd try that.