Heightmap and level streaming

I have a sizable world and, to conserve resources for development, want to apply level streaming. I’ve read up on level streaming and watched quite a few on-line tutorials, I feel I still do not fully understand. What I’d like to do is set a box-target around an entire portion of vacant map - a place where a future city will be built - and then create a stream level that can be toggled on and off to conserve resources. The only item in that large sectioned off area at this time is a road. I created a new map with the trigger-box selected but the streamed level doesn’t toggle on and off (while the persistent does). Its obvious that I missed something or perhaps I’m just not understanding something as I should.

Is this issue because I’m using a heightmap? How can I correct whatever I did wrong to enable level streaming as I’d like to?

Thanks in advance