Diagnosis
The Highland's firebox and cook chamber doors aren't famous for a perfect seal from the factory. The gaps let air in, which messes with your airflow and makes temperature control impossible.
How to Fix It
You need to seal it. The go-to move is high-heat food-grade silicone gasket rope, like Lavalock. Clean the door edges with rubbing alcohol, let it dry, then run a continuous bead of the gasket adhesive (included with most kits) around the inner lip of the door frame. Press the rope into it, close the door, and let it cure for a few hours before firing it up. This creates a compressible seal.
If That Didn't Work
If you still have a major leak, check that the door itself isn't warped. You can sometimes adjust the latch mechanism to pull it tighter, but a warped door might need more persuasion (gentle bending with a mallet and block of wood) or a warranty claim.
Heads Up
Don't use the smoker while the adhesive is curing—the fumes are nasty. Also, after sealing, your airflow control changes. You'll need to use the intake damper on the firebox more precisely since less air is sneaking in elsewhere.