I have a GTX 1070 from MSI with 3 DisplayPort, 1 HDMI and 1 DVI port.
For most of the games I play (Osu! Mania, Minecraft, Wolfenstein, etc.) I can't use the full screen mode because my 2nd monitor (BenQ GW2760) starts to flicker and the whole game is over. Only when I unplug the HDMI cable in the monitor, the game was no longer. Special cases where I do not have to do that, for example. Sea of Thieves, CSGO, Dead by Daylight and Overwatch.
Main monitor (Asus ROG XG32VQ) connected with Displayport in the PC and MiniDisplayport in the monitor
Go to the electrical shop of your trust and ask for it, I think nobody can help you here.