Bought Samsung NU8000 TV....cannot select 1440p on XBox One X nor see Frames Rate drop down ...

It is supported. There have been issues getting it to work. It depends on what other options you have turned on or off within the T.V. and X.

Hi

Your Xbox One will only offer the options of 1440p and 120Hz if your television supports them.

https://support.xbox.com/xbox-one/console/adjust-display-settings

Hi,

The Samsung NU8000 is a 4K UHD TV, you won't see 1440P on this display as it's not a supported resolution for this TV.

Even if 1440P (or 2,560 x 1,440) was supported, you won't get better performance on the One X as all enhanced games run at 4K regardless of what resolution is enabled in the options menu. It would only be down-scaled.

Is this what you were looking for, or do you need more help?

I dont think the Xbox One S can do 1440p at 120hz, only 1080p at 120hz. at 1440p you'll only see 60hz.

Below is a little more information that might help.

120Hz is only available for 1080p and 1440p. If you change your resolution to 4K UHD or 720p, the refresh rate
will be set to 60Hz by default. If your console is set for 4K UHD and you change your refresh rate to 120Hz, your resolution will be downgraded to the highest resolution available
for your display.

To take advantage of 120Hz, you'll need an Xbox One X or Xbox One S running the Xbox May 2018 Update. This update is currently limited to a subset of users enrolled in the Xbox Insider Program, ahead of a wider release set for the coming weeks.

Your Xbox One must also be connected to a compatible TV or monitor, which supports refresh rates over 60Hz. High refresh rates must be supported over HDMI 2.0, due to limitations of the Xbox One hardware.

Constraints of HDMI 2.0 also means 120Hz support cannot be used in conjunction with 4K resolution. And bandwidth limitations mean 1440p or 1080p resolution must be used, cutting a few pixels for those extra frames.

But even with the feature, don't expect system-wide improvements. Many Xbox One titles are developed with hardcoded frame-rate limits, set between 30Hz and 60Hz. While some games are developed with uncapped frame rates, expect many games to see no changes.
We recommend researching your favorite titles for details on frame rate settings.

Because of this resolution drop, the feature isn't recommended for people many with 4K displays. With few games offering 120Hz support on the console, the benefits are limited. But for 1440p or 1080p users, there are advantages.

How to enable Xbox One 120Hz support

To get started with high refresh rates on Xbox One, you'll need to enable the feature on your console. After installing the latest update, follow these steps:

1.Navigate to the Settings app on your Xbox One.

2.Choose the Display & sound tab

3.Select the Video output tile.

4.Open the Refresh rate drop-down menu.

5.Choose 120Hz to increase your refresh rate. (Your console should now output a 120Hz signal.)

I would look into your monitor and call DELL not Microsoft. I also have a One X and have never had any problems, as soon as select 120hz it automatically drops me down from 4K to 1440p. You must be misunderstanding something about your monitor as there
must be a defect of some sort or your monitor does not support 1440p at 120hz.

For reference, before responding I booted on both my One X's. One is connected to a Samsung 4K QLED and I had no problem switching from 4k 60 to 1440p 120. My other X is connected via an LG 4K monitor and again when selecting 1440p OR 120hz it automatically
selects 1440p or 120hz without any issues.