The XP-400 is listed on Apple's list of AirPrint printers, so it should work as soon as you pair it with your WiFi. No other printer installation is involved. I assume your XP-400 has the latest updates from Epson.
Are you using an Epson printing App, by any chance? The Epson printing App does not use AirPrint, it uses the printer's native printing language. Other Apps on the iPad can not print via the Epson App.
In other words, something else is keeping your iPad from seeing the printer. One cause might be that your WiFi router does not correctly handle Bonjour data required by AirPrint. If that is your problem, contact the router's manufacturer to see if there is a change in its setup or a firmware update that will solve the problem.