Without delving too deeply into to technical complexities of TLS/SSL, each time that you power-cycle your printer, it automatically generates a new self-signed Certificate with which to encrypt the printing connection. Being self-signed and without a trusted-root, your iPad (and other devices) cannot validate the authenticity of the Certificate - and for this reason you will see the warning that you illustrate. That said, selecting Continue should allow you to click-through any associated security warnings.
If you still cannot print over AirPrint, some other network issue is likely to be the cause. 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) (if any)
- WiFi Access Points / Network Extenders
- Other fixed infrastructure devices - such as Printers
- Client devices (your iPad)
Restarting your iPad, after restarting other devices in the correct sequence, is most simply achieved by a forced-restart of the iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
If you continue to have difficulties, check to see if your iPad is using the new WiFi Private Address feature. This feature is intended to provide additional privacy and security when using “public” (untrusted) WiFi networks.
Settings > WiFi > [Your WiFi Network / SSID] - tap the “i” icon - Private Address
The Private Address feature is largely unnecessary for your home WiFi network; if enabled, Private Addressing can interfere with anticipated operation of other devices, such as AirPrint/Airplay. In such circumstances, you may need to disable this feature - and can be enabled or disabled for each individual network.
Use private Wi-Fi addresses in iOS 14, iPadOS 14, and watchOS 7 - Apple Support
