Installed 15.0 to iPad Air, will not connect to printer says printer is off line but is on line

First, ensure that your printer supports AirPrint - and that it has connected to the same WiFi network as your iPad. When correctly set-up, your iPad should automatically detect your Printer.

If set-up correctly, should you continue to experience difficulties, your WiFi Router may be unexpectedly failing to pass Apple Bonjour traffic on you local network - or your WiFi Router or Network Switch may be blocking this device discovery protocol.

The Apple Bonjour protocol uses mDNS (Multicast DNS) UDP port 5353.

You may find that restart of your network components will resolve the problem. You’ll need to restart your network devices in the correct order. Assuming that you WiFi Router provides DNS services for your network, restart devices in this order - allowing time for devices to fully boot/restart prior to restarting other devices:

• WiFi Router
• Upstream wired Network Switch(es)
• WiFi Access Points / Network Extenders
• Other fixed infrastructure devices - such as Printers
• Client devices

Restarting your iPad, after restarting other devices in the correct sequence, is most simply achieved by a forced-restart of the iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

• Turn off your printer
• Turn off all other devices connected to the same wifi network (if you are using VPN connections, disable them)
• Turn off your modem-router and wait for 1 minute. Then turn it back on and wait for its complete autoconfiguration
• Turn on your printer, checking if it is connected to wifi
• Turn on and re-connect all other devices.

Regards

Giulio