We use Apple's built-in Airprint technology, so there's 1 main reason why it's not showing up: The printer might not be Airprint capable.
Wi-Fi ready doesn't always mean it has Airprint capability. You can check Apple's official list of printers here or be sure to check the box of the printer.
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"My Printer is Airprint Ready"
If your printer has Airprint, then it might not be connected to the same local network as your iOS device. The printer needs to connect to your router first. That's how the printer is kept secure as well. Once it's connected, your iOS device should be able to see the printer.