Played Death On Prime Time today as a chill out before going to work and I had to redo this mission several times because there seems to be a hole in the ground when going towards the stronghold.
In the end I decided to just land a helicopter on the roof because I kept falling through the ground, and also saw enemy cars underground there.
One spot is if you follow the road where you respawn when you die. But I also fell down right at the base's entrence.
Playing on PlayStation 4.