Yes you can use your iPhone’s hotspot, you don’t need to bring a laptop. Though, it may be easier to just set up a wireless router there, so that when you are on site you can use any device to connect to the same router and communicate with OpenSprinkler that way. The reason is that the firmware performs checking periodically to see if it’s still connected to the router, or otherwise it will attempt to reconnect. Having a router there makes sure it doesn’t lose connection when you take your iphone away. Wireless routers these days can be purchased at about 10 bucks, and you don’t have to provide Internet access to the router.