HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

HoMM3 HD - the mod does not affect either the installation of your game (all your files in the Heroes folder remain intact), or internet connection settings; only the game interface is changed. All problems stem from the basic game installation and operating system (network settings, as a rule).
Some information about the problem can be found in useful links. You need to install version Heroes 3.2 first, and then the mod.
And don't do that :)
The described problems are presented somewhat chaotically and vaguely, and they can be discussed with one of the players via ICQ if the available published information is insufficient.
p.s. Regarding difficulties with connecting/transferring data in Gameranger, it was discovered that when updating the antivirus software, its core (system functions) may also have been updated (virtually all antiviruses today incorporate firewall functions; and vice versa). Therefore, you should roll back the updates or check the antivirus functions.

It was after installing the MOD that the problems started. When I installed it for the first time, there was an option to change the resolution, which I did, and it changed and now looks like this. While reading about possible drawbacks of the MOD on the forum, it was written that it might not display in full screen under certain conditions, which seems to have happened specifically to me. I decided to revert to the original resolution, but further changes prevented the game from launching in full-screen mode. The resolution itself changed, but it was smaller than the entire screen. The game image relative to the screen became larger or smaller against a black background. The maximum possible resolution was 1024*768, even though I set a higher value in the mod settings. "Resolution" (meaning the pixel ratio of the matrix) of my screen is 1280*800, so when starting the game, there are black bars of 128 pixels on both sides.

But the point is that after completely uninstalling the game and the MOD, a new installation of the game gives the same results as before deletion, i.e., the same non-full-screen display. And even a completely separate installation of the complete collection gave the same effect.
So, it's 100% caused by the MOD.