Due to is used with a noun entity
whereas Because of is used with a verb phrase
For eg
The meeting was cancelled because of unavailability of the president (Here the action - meeting was cancelled happened )
the cancellation of the meeting was due to unavailability of the president ( Here the cancellation which is a noun , happened)
the easiest way to check is to use caused by
Whenever you encounter Due to replace it with caused by , if the sentence still makes sense then the usage is correct
For eg in the above example
The cancellation of the meeting was Caused BY unavailability of the president- Makes sense
The meeting was cancelled caused by unavailability of the president . ( Doesn't make sense)
Hope it helps