According to Netflix help page and NVIDIA help page, you need to meet the following requirements:

Netflix
A Windows 10 computer or tablet with the latest Windows updates installed.
- If you are not running Windows 10, you can forget the rest.

The Microsoft Edge browser or the Netflix app for Windows 10.
- Get the Netflix app from Microsoft Store in Windows 10. If Microsoft Edge browser, make sure it is the Chromium-based version of the browser.

A 60Hz 4K capable display (with HDCP 2.2 connection if external display). (Note: Every monitor connected to your computer must meet these requirements to successfully stream in Ultra HD.)
- Your Dell S3221QS meets the requirement. 4k capable device. Input: 2× HDMI 2.0 (with HDCP2.2 compliance); 1× DP 1.2;

Intel's 7th generation Core CPU (i3, i5, or i7 models in the 7xxx or 7Yxx series) or newer, or a NVIDIA GPU that meets these requirements.
- Your processor doesn't meet your requirements but your GPU is. It states either. So, Netflix will use your GPU for decoding.

A Netflix plan that supports streaming in Ultra HD.
- If you are not subscribed to the highest available plan, then don't look further.

A steady internet connection speed of 25 megabits per second or higher.
- Wired connection preferred to avoid drop in connection quality.

Streaming quality set to Auto or High.
- Settings inside Netflix app or if browser, the Settings page.

Another thing, if you are running multi-monitor setup, please refer to the NVIDIA link above.

So, your issue stem from using the DP port on your GPU to connect to the monitor via the supplied DP cable. DP 1.2 is only compliant with HDCP1.3.

Solution: Get a HDMI 2.0 compliant cable. And make sure you get a quality cable because of the bandwidth requirement for 4K is high.