The problem was even worse when I used the original HDMI-HDMI cable. The monitor will go black every 5,6 hours and come back with no signal. Have to reconnect the cable to recover. That's why I bought the DP1.4 cable. But I fixed the problem (I hope so) by doing this: I changed the scaling of extend monitor from 175% to 150%(recommended) and it stopped flickering. My main monitor's scaling is 125%, the recommended is 100%, it works well. The system recommended scaling is too small on 4K resolution. But why the scaling setting makes the monitor flicker?
Desktop: Alienware Aurora R11 GPU: NVIDIA(R) GeForce RTX(TM) 3070 8GB GDDR6 Main display: AW2721D  (set to 2K  144hz) Extend display: S2721QS  (set to 4K 60hz)   Now the extend monitor keeps flickering. Go black every 20, 30 seconds, and recover in 1 second. I think it's not the monitor's problem because when I run the self diagnostic on the monitor, it stops flickering. I had the newest drivers, bought a new DP1.4 cable, but doesn't help.
Aurora R11, extended monitor flickering every 30 seconds