not sure if you came right with this or not :)
If your looking to maximize your fps you can do the following on your series s|x console :
Go to Console Settings > General > TV & Display options > Video Fidelity and Overscan
Click on Display Overrides should be default to Auto - Detect click to change it to HDMI it should give a pop up just click Use Manual Settings.
This might make your display at lower resolution but not to worry just go back to previous menu and you should now have the option to change your Display Resolution and FPS.
Here you can set your resolution to 1440p and your Refresh Rate to 120 Hz.
Hope this helps :)