To fully uninstall the web plugin go to Control Panel\All Control Panel Items\Programs and Features and uninstall IPCWebComponents.
Within IE you are on your own. In Firefox there is a way to uninstall the plugins. Then restart your computer. The first time you try to login to the camera it will indicate the plugin is not found and provide you a link to download the executable file to install. Be sure you have shut down IE before you uninstall or reinstall the plugin.
From the screen shot of the camera tool it looks like you are using the correct IP address. I use different ports for each camera but it should not make any difference. Are your cameras hard wired or are you using wireless?
How long has it been since you shut down power off for everything, computer, cameras, and router? When you are making changes a full cold restart is advisable after a change is made. Are both cameras still visible from the desktop computer?
My next suggestion is to install a different web browser, such as Firefox, and reinstall the plug in then try to access the camera.
Have you updated the firmware on either of the cameras? Not suggesting you do so, just asking if you have done so.
Go to the computer where you can successfully access each camera. Look at the Device Information section under Settings -> Status. On each camera the bottom three lines should match. See attachments.