Samsung QN900D Neo QLED 8K TV game mode, 120 Hz or VRR is not working

The checks in more detail

Start at the box. Plug the console into an HDMI 2.1 port on the One Connect box, and use a certified ultra high-speed HDMI cable, which supports 48 gigabits per second; a cable that is not up to it limits the signal. Check that the cable from the box to the panel is firmly seated, since a loose One Connect cable causes dropouts. In the TV's settings, open General, External Device Manager, Input Signal Plus, and enable it for that HDMI port, which turns on the enhanced HDMI mode for 4K 120 hertz and VRR.

Without Input Signal Plus, the port runs in a basic mode. Then Game Mode. Under General, Game Mode Settings, set Game Mode to Auto or On, and make sure that VRR is on and that Game Motion Plus is off, since it interferes with VRR. The Game Bar, opened by long-pressing Play or Pause on the remote, shows the live refresh rate and the VRR status, so use it to see what the TV is receiving. On the console, set the resolution to 4K and the refresh rate to 120 hertz in the video settings, enable VRR and, for an Xbox, Auto Low Latency Mode.

If the console reports that the TV does not support 120 hertz, the cable or the port setting is the cause. Update the TV software in Settings, Support, Software Update. Note that the QN900D has no Dolby Vision, so a console set to Dolby Vision gaming will not use it. If the signal drops, try another HDMI 2.1 port on the box and another cable. Contact Samsung with the model number if the box is at fault.