cant get 120hz

I believe that you need to have a monitor that is Freesync and Freesync has to be supported over HDMI, as well as, the monitor needs to have HDMI 2.0 ports and supports 120/144hz, looking at that, and being G-sync, it seems to have HDMI 1.4 ports which limits
it to 1440p at 60hz

A new monitor is needed if you want 1440p at 120hz,

Pulled this info from a website...

"Nowadays, most of the monitors, TVs, and graphics cards feature at least an HDMI 1.4 port which is enough for 144Hz at 1080p, 75Hz at 1440p, and 30Hz at 4K.

However, keep in mind that some monitors have limited bandwidth over HDMI 1.4 making it max out at 120Hz or even at 60Hz in some cases (mostly G-SYNC and older monitors).

In fact, most 144Hz monitors are limited to 120Hz over HDMI 1.4, but some monitor manufacturers will use custom timing parameters, pixel rates, etc to use the full potential of the HDMI 1.4 bandwidth thus getting 144Hz at 1080p.

So, make sure that the exact monitor you’re looking at supports 144Hz or at least 120Hz over its HDMI 1.4 port – or just look for a 144Hz display with HDMI 2.0.

HDMI 2.0 is also fairly standard and can be used for 240Hz at 1080p, 144Hz at 1440p, and 60Hz at 4K. The latest HDMI 2.1 adds native support for 120Hz at 4K UHD
and 60Hz at 8K."

Here the link to the page..

https://www.displayninja.com/which-cable-do-i-need-for-144hz/

The problem could probably have been caused by the hdmi cable itself, you could try to use an hdmi 2.0 verified cable

The display only has HDMI 1.4 so it means that it supports 120Hz by using Y′CBCR with 4:2:2 or 4:2:0 subsampling. You should check the settings from your display.

If it would have HDMI 2.0 it would have no problem with the setting.

This option is usually auto set by your Xbox do to your HDMI cord nor monitor sending a signal that the Xbox notices and makes changes off of that! you SHOULD be able to override that doing the following System > Settings > Display
& sound > Audio output and choosing from HDMI audio, optical
audio, or Bitstream format.

What kind of tv do you have? Ive seen some work but i think if your monitor cant get set directly to 120hz through the settings then it may not work