Hi,
Also, if all Wi-Fi settings are OK, but there is a problem with device discovery, you could try the following power-cycle sequence (with these waiting times; do not forget the router): Power off the Wi-Fi router, the printer, the iPad, and all other connected devices/computers. Wait two to three minutes. Power on the router. Wait two to three minutes. Power on the printer. Wait a minute or two. Power on the iPad. Finally, power on all other devices/computers.
It is worth noting that the WF-4820 has an Ethernet port, in addition to Wi-Fi (and USB). AirPrint works with wireless (Wi-Fi) or wired (Ethernet) network connections. This means that you could choose to connect the WF-4820 to one of the LAN ports of the Wi-Fi router (instead of using a wireless connection). An Ethernet connection is often more stable. The iPad would, of course, still use a wireless connection to the Wi-Fi router.