Install CubicChunks first: https://minecraft.curseforge.com/projects/opencubicchunks
Dungeon generator will generate dungeons according <world_save_folder>/data/labyrinth/labyrinth_level_preset.json file.
Download example at
"minX", "maxX", "minY", "maxY", "minZ", "maxZ" define area in cube size units, where generator will replace cubes. To convert coordinates to block units, multiply them to 16.
"generator_type" define a set of cube shapes and rules of placing. There is 4 types: "basic", "claustrophobic", "village" and "lava".
"mapping" define blocks which will be used to place shapes in world. "walls" for walls, "floor" for floor etc.
You can peek a list of keys here: https://github.com/Foghrye4/Labyrinth/blob/MC_1.12/src/main/java/labyrinth/worldgen/LevelsStorage.java#L34
You don't need to install this mod at client side while connecting to server with this mod installed.
Note that "Labyrinth" is not actually a labyrinth. It just randomly generate continuous walls next to each other, so it is possible to find completely sealed rooms or huge open spaces.