Tutorial:
1. Open Program HxD > Click File and Open ds.exe
2. Ctrl+R > Click "Hex-values" > Click Search Direction "All" > Write 55 55 15 40 in Search for,
3. Replace with Hex value of your Resolution's Aspect Ratio , Here are some common resolutions:

2560x1080 = 26 B4 17 40
3440x1440 = 8E E3 18 40
5120x2160 = 26 B4 17 40
6880x2880 = 8E E3 18 40
3840x1080 = 39 8E 63 40
5120x1440 = 39 8E 63 40


4. Click Replace All


5. Save and play

Maybe change in the settings from 21:9 to 16:9 and than back to 21:9.