BACKUP Fog.dll, Game.exe, Patch_D2.mpq for future uninstall.

D2SE: Extract into Mods as usual. Realm is automatically set.

Normal user: Extract files into game folder.
Standard client realm connection:
1. Extract bngatewayeditor.exe anywhere on your computer
2. Open it and choose Diablo2/Diablo2Exp window on the bottom
3. Select Add Gateway
4. 
Name : Reckoning
Zone : 0
IP: 217.182.215.18
Press OK
5. Click on the new choice of gateway.
6. Press Set to Chosen Gateway

Website: https://diablo2.dev