I have a SAMSUNG 49” CRG9, which supports 5120 x 1440 @ 120hz. It works perfectly with on MacBook Air 2020. However, on the new Mac Mini M1 I had a hard time getting 5120x1440 @ 120hz. At first the M1 wouldn’t work with the DisplayPort to USB-C cable, even after updating to Big Sur 11.0.1. I assuming it’s because the usb-c cable I had only supports DP 1.3. The HDMI cable worked but not at 5120 x 1440.
I ordered a Maxonar DisplayPort to USB-C cable from Amazon, which supports DP 1.4. I was able to connect the monitor with the new USB-C cable, however, max resolution I was able to get was 3840 x 1080 @ 60hz. I had to change the refresh rate setting on my monitor from 120hz to 60hz and turn off Freesync. That is when I was finally able to get 5120 x 1440 (low resolution).
I am still having a problem where the system won’t detect the monitor when waking up from sleep. Currently, I have to unplug and replug the USB-C for the monitor to connect. It might just be a problem with the DisplayPort drivers or Big Sur. I am going to try order different DP 1.4 cables to see if that fixes the problem.