Each TFD license includes an unlimited number of render node licenses. You simply don’t have to activate a license for a render node at all.
The most likely cause of headless renders not working is that the node cannot load the cache files. You should have error messages in either the Deadline log and/or the TFD log (see below about where to find the log).
The cache path set in the object parameters of the TFD container will also be used by a render node. Therefore, the path must be valid for this machine as well.
For the case where the path used to store the cache during simulation cannot be a valid path on the render node, you can set the Additional Cache Path. This avoids having to repeatedly change the path when sending a scene to the render farm. Once both paths are stored in the scene, the sim will always use the first, while the render nodes will try the first, fail, and then try the second path.
For example, if you store the cache to a fast local drive during simulation (recommended), it likely has a local path like D:\TFD Caches\Cache 001. The render node cannot access this path. You or Deadline have to copy the cache folder to a network drive like \\file_server\scene001\tfd_cache001. Those two paths would then be the primary and secondary cache paths set on the TFD container.
The TurbulenceFD log file can be found at
(Windows) %USERPROFILE%\AppData\Roaming\jawset\turbulence.log
(MacOS) ~/Library/Preferences/jawset/turbulence.log