The amendment says “their respective numbers” the invaders are not theirs and must not be counted.
Would you count the members of the Mexican army if Mexico officially declared war on us and occupied Arizona?
NO, you would not.
The civilian invaders are not different, they must not be counted and their children DO NOT receive citizenship just because they are born on US soil.
Read as a whole, the illegal immigrants can be discounted initially as transients, or counted and then discounted as being neither citizens nor legal residents. Either way they must be identified, and in neither way are their numbers to be used for apportioning representation.