In a large cast-iron skillet over the campfire, heat oil until bubbles form. Place sugar and cinnamon sugar in a brown lunch bag and shake to combine.
Poke a hole in the middle of each biscuit half (like a donut). Fry biscuits until golden, 3 to 4 minutes per side.
Drain donuts on paper towels, then add to paper bag and shake until fully coated in cinnamon sugar.