There is definitely a disconnect between our speaker and his neighbor. They work together to mend the wall, but they don’t talk to each other as they go along. The speaker wishes to put a "notion" in his neighbor’s head, but he doesn’t actually attempt to challenge his neighbor’s love of the wall. The wall takes on greater meaning as we watch the lines of communication shut down between the speaker and his neighbor.
The wall helps to open up lines of communication between our speaker and his neighbor.
The wall is a metaphor for the wall that blocks communication between our speaker and his neighbor.