Hello ,
UGameUserSettings* MyGameSettings = GEngine->GetGameUserSettings();
MyGameSettings->SetFullscreenMode(EWindowMode::Fullscreen);
MyGameSettings->SetScreenResolution({ 1920, 1080 });
MyGameSettings->ApplySettings(true);
MyGameSettings->SaveSettings();
This is another way of doing that. You can set your desired resolution and fullscreen at runtime. These settings are saved in .ini file.
XadE