Always keep in mind what the "kingdom of Heaven" meant to them. Do not allow the WTS's depiction colour your thinking.
For Jesus, the Kingdom was an imminent, practical, visible era that had already commenced. The kingdom was totally fixated on the Jews, which would see them released from subservience and they would be restored to their rightful position.
For Matthew 18 in particular, keep in mind that the purpose of the Gospel is to prove to (other) Jews that Jesus was indeed their promised Messiah; and for Jews this meant a strong warrior king who would overthrow their enemies. The problem for the Jews was that rather than defeating the Romans oppressors, Jesus was killed by them.
The sole message of the Gospel of Matthew was to prove to Jews that Jesus fulfilled the expectations set out in their own writings. The only requirement of doctrinal belief set out in that Gospel is the need to completely observe all of the (OT) Law and Prophets to the letter (and beyond), driven from within, not for reasons of external show.
Doug