Hi,
Would it be correct to assume that your printer is a model supporting the AirPrint technology (About AirPrint - Apple Support)? If so, a special app from the printer manufacturer should not be needed for the iPhone.
The AirPrint-enabled printer must be connected to the same network (Wi-Fi router) as the iPhone. If the iPhone cannot properly detect the printer, you could try a full power-cycle sequence (do not forget the router): 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.