I downscaled the terrain to 4k and removed most unnecessary assets. For anyone having trouble with missing terrain, please try running the attached map and let me know if the terrain is visible. Make sure to remove all other copies of the map first, and clear your cache (may be done by deleting BeamNG.drive/0.33/temp/levels/river_highway or by using the official clear cache function). The map should look something like this:
Which version of the game are you using? Someone else with this issue found it is working with v0.34. It'd also be useful to know your computer's specs.