DIAGNOSIS: This is normal. The window is not thermally insulated. The temperature difference between inside cooking steam and outside room air causes condensation. It is a design flaw.
FIX: There is no permanent fix. You can try preheating the empty air fryer for 2-3 minutes before adding food. This reduces the initial thermal shock. Wiping the outside of the window with a drop of dish soap, then buffing it off, can reduce surface tension and fog slightly.
IF THAT DIDN'T WORK: Accept that the window is largely decorative. Rely on the timer and occasional shaking, not visual checks. Do not attempt to clean the inside of the window; it is not removable.
HEADS UP: Never spray glass cleaner or any chemical on the window while the unit is hot. Thermal shock can crack the plastic lens.