Community transports are interchangeable transport layers for the Netcode for GameObjects package.
Transports can be installed with the Unity Package Manager. Please follow the instructions in the manual about Installing a package from a Git URL.
Use the following URL in the package manager to add a transport via git URL. Change com.community.netcode.transport.enet to any of the packages in the Transport folder to choose which transport to add:
https://github.com/Unity-Technologies/multiplayer-community-contributions.git?path=/Transports/com.community.netcode.transport.enet
After installing a transport package the transport will show up in the Select Transport dropdown of the NetworkManager (To make the dropdown appear set the Network Transport field to none first in the inspector)