Do the following, in this order:
- If you have VPN installed, delete the VPN profile and app - If that doesn't fix it proceed with the next steps
- power off the printer
- Power off your iPhone
- Restart your router by unplugging it for 15 seconds (many newer routers have no true power switch)
- Turn on your iPhone
- Connect the iPhone to the Wi-Fi network if it doesn’t automatically connect
- Power on the printer and wait for it to connect to Wi-Fi
You should also check with the printer manufacturer for a software update for the printer.
These steps restart the BonJour network that supports AirPrint, and will usually resolve any AirPrint issues. You probably don’t need to do all of them, but following these steps is usually less work than finding the one that is causing the problem.