My iPad has stopped identifying my router and says the WiFi is not detected. I have no problem with my iPhone.

Start here:

If your iPhone, iPad or iPod touch won’t connect to a Wi-Fi network – Apple Support

A restart of your WiFi Router may also resolve the problem. To restart you home router, disconnect power - then wait at least 30 seconds before restarting. When you router has rebooted (which may take several minutes), force-restart your 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.