Unable to reconnect my iPhone to printer after iOS 18.4 update?

So, when you want to print is your printer showing or selectable? If not then it’s likely a networking problem. If your printer is shown what happens when you tap ‘print’? We’re not mind readers and you need to explain in detail what is happening, please.

Try restarting the iPhone, printer, and Wifi router. Test

This happens from time to time and when it does, I do exactly what lkrupp advised and it resets the connection EVERY time. Another thing I encourage you to do after EVERY iOS update is to Force Restart your iPhone. See how to do that here --> Force restart iPhone - Apple Support

This has nothing to do with the iOS update. Your device is probably broken. Take your device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support:

See the pic below which is self-explanatory. Printing a password-protected PDF on an AirPrinter.