Apply double-sided adhesive tape on the backside of the front camera. Adhere the dot projector flex cable to the right position.
Warning: the double-sided adhesive tape used here is for test. We need to glue the dot projector module onto the flex cable after fault clearance.
Now we can assemble the phone and test. Go to Settings > Face ID & Passcode > Set Up Face ID. Face ID can be setup successfully. The phone can be unlocked with Face ID this time.