Hi,
When a printer model supports the AirPrint technology (About AirPrint - Apple Support), a special app from the printer manufacturer is not needed for iPhones/iPads. The AirPrint-enabled printer must be connected to the same network (Wi-Fi router) as the iPhones/iPad, which I assume is the case here.
If the iPhones/iPad cannot properly detect the printer, you may want to test a full power-cycle sequence (do not forget the router): Power off the Wi-Fi router, the printer, the iPhones, the iPad, 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 iPhones and the iPad. Finally, power on all other devices/computers.