If the problem is still not fixed after that then it may be because you have a G-Sync panel. Some people report such models cannot be switched to use the integrated GPU but I would still check Nvidia Settings to see if you can disable G-Sync. Overall this is a Dell hardware design issue, and also an Nvidia driver bug for not offering better support for this hardware arrangement.
Some googling suggests:
1. Press Fn + F7
2. Reboot.
If the problem is still not fixed after that then it may be because you have a G-Sync panel. Some people report such models cannot be switched to use the integrated GPU but I would still check Nvidia Settings to see if you can disable G-Sync. Overall this is a Dell hardware design issue, and also an Nvidia driver bug for not offering better support for this hardware arrangement.