Can you print to it from the Mac Mini? If not, the printer may not be connecting to the network. If it is, you should be able to check for a printer update from its control panel. It should at least tell you if it is up to date.
The most likely problem is that your apartment WiFi system does not correctly handle Bonjour data required by AirPrint. There may be a change to its setup or a firmware update that will fix the problem.
I can see that you may not be able to do anything about a problem with your apartment WiFi. The only way around it I can think of, is to purchase you own private WiFi router just for printing. You would need to pair the printer to this WiFi router, and change your iPad or iPhone temporarily to it when you want to print. Perhaps an Apple AirPort Express would work for you.
Note: Do not connect your printer to the WiFi using USB, because AirPrint does not work via USB.