Hi,
If there is a problem with AirPrint discovery for one or more devices, and if all settings are correct, you may want to try the following full procedure (in the exact order):
Power off the Wi-Fi router, the printer, the iPhone, and all other 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 iPhone. Finally, power on all other devices/computers.
About AirPrint - Apple Support
Use AirPrint to print from your iPhone, iPad, or iPod touch - Apple Support