Recently I faced similar issues with 2 of my new monitors. Both were Samsung S2Nf350 Slim series LEDs (S22F350 & S24F350) & the issue was same.
After a long sleep/idle time the monitors will not wakeup with a mere keypress or mouse movement. I had to reach to the back of the monitors and press the power on button to awake the monitors!
I had chats with Samsung support & they asked me to take the monitors to nearest workshop as the LEGENDS at 1st level support decided that it was a hardware issue.
Then I decided to go through the user manual & came to an area where it narrated about a feature called “Off Timer”. This MUST be a new feature that is available with new generation LED monitors, helping them to turn off automatically after a pre-set period (default 4 hours)
Turning the feature to OFF state has resolved my weeks long “issues” with the monitor not awakening from a sleep.
May be that’s your solution also.