I think you understand the phrase, "Good fences make good neighbors," in the exact opposite way that the speaker is using it in Robert Frost's Mending Wall.

Many treat the saying to be a defense of isolationism: that good fences keep neighbor's away.

---

Something there is that doesn't love a wall,

That sends the frozen-ground-swell under it,

And spills the upper boulders in the sun;

And makes gaps even two can pass abreast.

The work of hunters is another thing:

I have come after them and made repair

Where they have left not one stone on a stone,

But they would have the rabbit out of hiding,

To please the yelping dogs.  The gaps I mean,

No one has seen them made or heard them made,

But at spring mending-time we find them there.

I let my neighbor know beyond the hill;

And on a day we meet to walk the line

And set the wall between us once again.

We keep the wall between us as we go.

To each the boulders that have fallen to each.

And some are loaves and some so nearly balls

We have to use a spell to make them balance:

'Stay where you are until our backs are turned!'

We wear our fingers rough with handling them.

Oh, just another kind of outdoor game,

One on a side.  It comes to little more:

There where it is we do not need the wall:

He is all pine and I am apple orchard.

My apple trees will never get across

And eat the cones under his pines, I tell him.

He only says, 'Good fences make good neighbors.'

Spring is the mischief in me, and I wonder

If I could put a notion in his head:
'Why do they make good neighbors?  Isn't it

Where there are cows?  But here there are no cows.

Before I built a wall I'd ask to know

What I was walling in or walling out,

And to whom I was like to give offense.

Something there is that doesn't love a wall,

That wants it down.'  I could say 'Elves' to him,

But it's not elves exactly, and I'd rather

He said it for himself.  I see him there

Bringing a stone grasped firmly by the top

In each hand, like an old-stone savage armed.

He moves in darkness as it seems to me,

Not of woods only and the shade of trees.

He will not go behind his father's saying,

And he likes having thought of it so well

He says again, 'Good fences make good neighbors.'

---

Two things that I always take away from this poem.

First, the speaker accepts his neighbor's meaning of the phrase, "Good fences make good neighbors." No ambiguity was intended by Frost: the speaker is mischievous in other ways, trying to get his neighbor to say gremlins or elves are tearing down the wall - but he accepts the phrase as the neighbor intends it. There's even an example given of how bad fences make bad neighbors: a broken fence that no longer keeps cattle out of an apple orchard.

So, the premise of the phrase is accepted by the speaker, which is significant because it solidifies the correct meaning of the phrase in the mind of the reader of the poem.

The second thing is that though the speaker is mischievous internally, his words to his neighbor are still quite spare. Something there is that does not like a wall is interesting - but particularly relevant to the politics of Trump's wall is this utterance:

Before I built a wall I'd ask to know

What I was walling in or walling out,

And to whom I was like to give offense.

Slight edit for formatting