You can start by double-checking the settings on your iPhone. Make sure Wi-Fi is connected to the same wireless network as the printer and it is preferable that Bluetooth be turned off.
If you are still unable to locate the printer, it could be a Bonjour issue with your router or access point. Download the HP ePrint Home and Biz app, it is a free app from the Apple App Store. Try to print from inside that app. If it is successful, then the Bonjour/ Multicast service may not be properly forwarded from your iPhone, through your routers, to your printer.
There are other steps you can take depending on the outcome of the steps above.
*I work for HP*
With hp eprint works, and works with all other devices, but not with my iDevices, iphone and ipod. All te devices are connected in the same network, and I don't have bluetooth. I'm pretty sure it's a problem with bonjour, becouse also I've a similar problem with renote, that can't find my itune library..
Well I think there's also a problem with the config of my ap or modem (some port I must open I think), but I don't know what I must do...
The D-Link DAP 1360, from what I can tell, does not have an option to turn Bonjour/ Multicast on or off and that could mean it does or does not support the service. I do not see it present in the interface, the link is below for an emulator.
As far as your modem, I assume it is a modem combination wireless router. What is its make and mode?
Some good first steps are to power down your iOS devices and printer, then unplug your routers/modem. Power your modem and routers back on fully, then power your iOS devices and printer on and try to AirPrint again.
*I work for HP*