Hi, we continue compiling with Mental Ray for Max 2017 and before, just to provide legacy compatibility. Even the latest Forest version (7.1.2) can use it.
But this is not the case for Max 2018, because Autodesk removed Mental Ray from it. If i remember well, Nvidia provided it as a separate package for some time.
But we never used this optional package. Therefore, there is no way to include support for Mental Ray in Max 2018 or greater.
Anyway, you could use Forest and convert it to standalone objects, using Forest Tools
There are some limitations (efficiency, number of renderable items, no Forest Color, etc), but it's doable as last resource.