The extended display might not be working due to incorrect display settings. So, first, make sure you have chosen the Extend these displays option under the Settings > System > Display. If the settings are good, there might be some other reasons behind the extended display not working. To fix it up, make sure you have installed all pending Windows updates and updated your GPU drivers. Other than that, you can also try disconnecting all peripherals and checking the cable connection that the monitor is properly connected to your system. If this doesn't work for you, you can try using a different port to plug in the secondary display on your PC.