Either set larger culling distance (click on Limit plane qty checkbox and set the new distance in kms by slider) or set the distance slider to 0 - that disables the plane culling entirely.
Thanks for the info JJJ. I didn't realize there was a slider as I was only 'unticking the box', never 'ticking it'.
On that note, I was hoping to just turn culling off permanently by unticking the 'limit plane qty' box but that didn't seem to do anything. Perhaps I was doing something wrong, or does mission editor limit 'plane quantity' whether the box is ticked or not?