PROBLEM SOLVED
Hi guys!
Long short story. I finally bought my own machine, which, because I am a geek, I assembled myself. OC is not an obstacle for me, so after the entire installation and after several unsuccessful attempts to enter the BIOS, I started to wonder why I spent so much money on equipment that I cannot configure myself.
However, after a long and tedious reading of all your posts regarding refreshing, fast boot, etc., I found the reason!
Well, in my case, having an Nvidia card and a Samsung monitor with 240 Hz refresh rate, it helped... changing the version of the DisplayPort connector! In order for me to use G-Sync, I must have the connector set to class 1.4. Changing to class 1.2 solves the problem!
However, what surprises me is that I tried to connect my different monitors using various methods, but it didn't work every time. Resetting the CMOS when removing the battery from the motherboard helped temporarily.
To sum up, thanks for the guidance and I hope my solution can be useful to someone!