Diagnosis
The oven's temperature sensor (that little metal probe in the back) is likely coated in baked-on grease or is faulty, sending the wrong reading to the board so it shuts off the heat too early.
How to Fix It
First, run the oven's self-cleaning cycle if you haven't. This can burn off residue on the sensor. If that doesn't help, unplug it, let it cool, and gently clean the temperature probe with a soft brush and a paste of baking soda and water, then wipe dry.
If That Didn't Work
You might have a failing heating element. Visually inspect the top quartz heating element for any obvious dark spots or breaks when the oven is cool. A partially dead element will still glow but not put out full heat.
Heads Up
These ovens are sensitive to rack position for air frying. If you're not using the top rack position for the air fry basket, you won't get proper crisping because you're too far from the top heating element and fan.