Apple devices typically use a service called Bonjour to look for AirPrint printers. Printers can be anywhere on your network and the service typically works to find out where (what IP address the printer is using). But if this doesn’t work for you, there can be a more reliable to connect to printers.
To set a particular printer to be accessed by an IP address, first you need to make that printer have a static IP (so that the IP address doesn’t change). This can be done through the printer itself or your router.
Then you need to make a profile using Apple Configurator (available through the Mac App Store). Make a new profile and name it something like “Printers.” Then add the printer(s) to the profile and put in the IP address(es). This profile can then be loaded on any Apple device (Mac OS or iOS).
By the way, this also helps if you want to print remotely while logged in through a VPN. Many VPN’s don’t play nicely with Bonjour, while using this direct approach works.