How do I add an air printer to my iPad. I had it but now it’s gone

Start here:

Use AirPrint to print from your iPhone, iPad, or iPod touch - Apple Support

As described, your WLAN Router/Switch may be unexpectedly blocking Apple Bonjour traffic on you local network.

The Apple Bonjour protocol uses mDNS (Multicast DNS) UDP port 5353.

You may find that restart of your network components will resolve the problem. You’ll need to restart your network devices in the correct order. Assuming that you WiFi Router provides DNS services for your network, restart devices in this order - allowing time for devices to fully boot/restart prior to restarting other devices:

• WiFi Router
• Upstream wired Network Switch(es)
• WiFi Access Points / Network Extenders
• Other fixed infrastructure devices - such as Printers
• Client devices