4K DISPLAY FRAMERATE SETTING

Hello,

With reference to my old case

Here I want to restart this discussion.
As in my old case I was using regular OS of Nvidia Jetson ubuntu 18.04.
I had my custom carrier board on which I am using Jetson Nano SOM.
Due to some design limitation I can’t able to support HDMI out 4K resolution at 60 FPS.
I restricted it to 4K resolution at 30 FPS.
In regular OS using unity-monitor.xml file I can restrict display to 4K@30Hz. Its working fine.
Now I am working on custom OS which has only command line utility(miniOS version) .
In this OS I tried same method of unity-monitor.xml file but its not working.
Then I searched on forum so I got few links on configuring xorg.conf file.
I tried those methods but it won’t work.
Please guide me on configuring xorg.conf file to restrict display to 4K@30Hz.
Thanks.