iPad consistently not connecting to Canon Pixma 8660 series printer

If you continue to experience difficulties, check to see if your iPad is using the WiFi Private Address feature. This feature causes your iPad's MAC Address to regularly change - the intent being 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

If you have a VPN App or profiles installed, or a security/AV/firewall App such as Norton, these may be interfering with your network connection. If applicable, the resolution is usually simple; you may need to disable or remove the Security App and/or VPN.

Otherwise, given your description and the steps already taken to address the problem, the logical conclusion is that your WiFi Router is unpredictably dropping multicast traffic.

In such circumstances - and assuming that your WiFi Router is the only network infrastructure component other than client devices and your printer - the only advice that I can offer is to (a) explore available settings of your Router, (b) reset your WiFi Router to factory settings, or (c) replace your WiFi Router in it's entirety.