Hi,
I have a problem with changing the resolution in the game. When I change and launch the game, it is always the same (poor). Thank you in advance for your help

Definition of poor? And what you mean stay same? There is some OSD display popup telling you which resolution used (some monitors/TV displays do that)?
You mean screen size stay same? There is few settings to that. in OS, in OS exe file Properties, in video driver control panel and in GOGs DXCFG tool. And sometimes in monitors OSD or driver settings if there is such additional driver or settings in OSD (some gaming monitors or TVs).

Default state is stretch to fill. It will stretch screen to fill wide-screen. Should be everywhere set to Keep Aspect ratio.

Stretching is EXACT what lead to POOR quality exactly. More blured and broken proportions. Old games lower resolutions not look poor on nowadays displays itself. It is stretching what make low resolutions be low-quality resolution. Resolution itself not directly related to quality.
You must set KEEP ASPECT in video driver with forcing checkbox and INTEGER scaling enabled.
Then go to GOGs DXCFG tool (or chose settings in GOGs Wc2 Launcher) and set KEEP ratio with Integer scaling options.

You get 640x480 stretched to 1280x960 or more depend on your display. With proper scaling it almost won't get bad poor quality on nowadays displays. Real resolution ingame still stay 640x480. GOG DXCFG not allow increase it. Technically you can use it to make wide-screen. No miracles - it will stay low resolution stretched to wide with blure and broken proportions. To do that disable in DXCFG Keep ratio and Integer scaling options. While keep Keep everywhere else.
If your display (more likely for TVs) have scaling options but predefined ones - you can use it to stretch screen a bit. For example if it is TV have some TV/DVD/Movies output standards to chose and have 16:9 output mode (along with ntsc/pal/4:3/zoom variations). If it have only chose between stretching and keep ratio (gaming monitors) - keep it as Keep ratio same as in system settings.

To make it wide-screen resolution that looks a bit better than GOGs DXCFG can make use 3rd party fan made mods. For Wc2 it is cnc-ddraw fork (while cnc-ddraw is fork itself) called Wc2 Wide-screen Hack mod. Released initially on war2ru forum.
Still no miracles. Games that initially 4:3 never get great wide-screen support. Best screen quality stay on stretched 640x480to1280x960. Wc2 Hack make game screen zoomed out which one also disadvantage. It is also increase overview of playable map area that is unbalanced advantage in Multiplayer and to not be disadvantage feature players should be sure everyone have same settings.

Go to GOG (not GOG Galaxy since you need browser to download mod) account games Library, Wc2, chose MORE - Support - Additional info on PCGamingWiki. There should be download link to wide-screen Hack. Or go directly to PCGW and search for Warcraft.

You need RENAME GOGs DDRAW.DLL file!

I ask for poor definition cause often people change their system settings to keep window small in central area and even call it "poor". And as I say there is "poor quality" real issue which one maybe did happen for you too. Maybe you already have proper settings in system. Any you can do now is change only GOGs DXCFG to wide-screen. And that mean you got wide but poor quality screen. Poor indeed. Maybe that one happen. Then skip changing system settings if you already did that previously and go to Wc2 Mod to increase screen quality at cost of changed FOV/zoom.

war2ru un official server support it and have additional icon in ingame UI to show host use modded version.

Or they drop that support? Initially players can have differ settings. Not sure if admin solve this unbalanced thing.

To get war2ru as addition to Global add their address (gateway) to Win REGEDIT. You can start normal War2BNE exe for Global and Wc2 Wide-screen Hack exe for Wc2ru server with this mod (not possible to play in un-modded MP session).

Wc2ru without this mod installed can be added to gateways and connected by normal Wc2BNE.exe. They support unmodded GOG as official version after it get released. Previously they require own client/exe to connect. Using their client add some multiplayer features (some tools). To un modded session on their server you no need mod your GOG version, as is good and keep Global available too.

if someone got problem he can ask and he should get help

Go to the game folder, open dxcfg.ini and adjust contents to match below:

[dxcfg]
display=desktop
presentation=application
aspect=disabled
scaling=fit
vsync=application
gamma=1.0
anisotropic=application
antialiasing=application

Alternatively use the "Video Settings" option from the launcher when you first start the game to adjust.

(anisotropic filtering and antialiasing can improve visuals, but on some systems it's better to leave them set to application - play with the settings and see how you go.)