Purchase Unreal Tournament: Game of the Year Edition on GOG (DRM-free, recommended) or Steam. The original CD also works. You need the full game files — not just the engine.
Download the OldUnreal v469d patch from oldunreal.com and install it over your UT99 directory. This is the community-maintained compatibility patch that fixes multiplayer on modern systems.
In the UT main menu: Multiplayer → Open Location → enter: