Hello, my problem is just as the title says: the game opens in the secondary screen (which I have set up vertically) instead of the main one. I have been trying everywhere in the options or properties but nothing seems to work.
Welcome here. Please try this. Press Alt + Enter to enter into Windowed mode. Now move the Window to the screen where you want to run the game and then press Alt + Enter again. It will go into full screen mode. Now press Alt + F4 to close the game. Now launch the game again. It should open on the correct screen.
This workaround is strictly related to AOE II DE. With the other AOE versions it might not work at all, e.g. AOE IV doesn’t support it. In that case the only option left is to make the external screen your primary one through graphic options. Probably this will even give you better performance, as dedicated GPU usually is associated with primary screen (if laptop has Intel/NVIDIA dual GPU onboard). Also notice the other two options.
Alternatively you could use the Win Flag + P key combination and select only external monitor to only instantly work on your secondary screen.