I can confidently say that while the basic models rely on Wi-Fi for internet access, there are exceptions. The Kindle with 3G/4G connectivity, like certain older models, can access the internet without Wi-Fi. These devices use cellular networks to download books, sync your library, or even browse the web, though the experience is slower and more limited than Wi-Fi.
However, most modern Kindles, like the Paperwhite or Oasis, are Wi-Fi only. Without Wi-Fi, you can still read downloaded books, use the dictionary, or take notes, but you won’t be able to shop for new titles or sync updates. If you’re often offline, downloading your books in advance is the way to go. The Kindle’s offline functionality is robust, but for full internet access, Wi-Fi or a cellular model is essential.