In the main towns and resort areas, Kauaʻi's cell service is fine for maps, calls, and streaming. But this is a rural island with mountains, canyons, and a wild coastline, and signal disappears in a lot of the places you will most want to explore. Knowing the dead zones ahead of time keeps a planning hiccup from becoming a real problem.
You will generally have solid coverage in and around Līhuʻe, Kapaʻa, Poʻipū, Princeville, Hanalei, and the main highway connecting them. That covers most hotels, restaurants, and the drive between them. All three major carriers work on the island, and most visitors report Verizon having the broadest reach on Kauaʻi specifically, though AT&T and T-Mobile are fine in the populated areas.
Expect little or no signal in the places that feel the most remote, because they are. Waimea Canyon and the Kōkeʻe area up top have spotty to no coverage. The Kalalau Trail and Na Pali Coast lose signal once you are past the first stretch. Keʻe Beach and the far end of the north shore are unreliable. There are also smaller dead spots in rural stretches, like the far end of the Tree Tunnel road near Kōloa. Plan as if you will have no signal at any trailhead or remote beach.
Download offline maps of the island before you go (Google Maps lets you save an offline area). Screenshot directions, reservation confirmations, and any addresses you will need. Tell someone your hiking plan and expected return time, since you may not be reachable on the trail. If you rely on data for navigation, a paper map or saved offline map is a cheap backup. For longer Na Pali or Kōkeʻe outings, treat your phone as a camera, not a lifeline.
Most hotels, vacation rentals, cafes, and many restaurants have Wi-Fi, so you can catch up on messages and check conditions when you are back in town. Coffee shops are reliable spots to get online. If you work remotely or need dependable connectivity, base yourself in a hub town and do not count on signal during day trips to the west side or the deep north shore.