Each remote control app — including Shutter — needs to be registered as a new smartphone device on your Canon camera. If you already use Shutter on your iPhone and want to add your iPad (or vice versa), you’ll need to go through the connection setup again for the second device. If you try to connect with the camera using a configuration of another app, the camera will reject the connection.
The exact steps depend on your camera model. Open your camera’s Menu and find the tab with network (wireless) options: