(C++) How to remove "isolated" elemenents in a 2D Array?

I am given a matrix which contains ones and zeros(randomly generated). I want to remove the "isolated" 1-s.

e.g. :

1 0 0 1 0
0 1 0 1 0
1 0 0 1 0
0 0 0 1 0

should be:

0 0 0 1 0
0 0 0 1 0
0 0 0 1 0
0 0 0 1 0

Every digit 1 which is surrounded by zeroes (diagonal doesn't count) must be changed to 0.

Here is what I have written so far but it doesn't seem to do anything (it prints back the same matrix as before):

void deleteIsolatedOnes(int(&digit_grid)[GRID_SIZE][GRID_SIZE])
{
    for (int i = 0; i < GRID_SIZE; i++)
    {
        for (int j = 0; j < GRID_SIZE; j++)
        {
            if (digit_grid[i][j]) // if the current element is one
            {
                if (i == 0 && j == 0) // top-left corner
                {
                    if (!digit_grid[i][j + 1] && !digit_grid[i + 1][j])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (i == 0 && j == GRID_SIZE - 1) // top-right corner
                {
                    if (!digit_grid[i][j - 1] && !digit_grid[i + 1][j])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (i == GRID_SIZE - 1 && j == 0) // bottom-left corner
                {
                    if (!digit_grid[i - 1][j] && !digit_grid[i][j + 1])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (i == GRID_SIZE - 1 && j == GRID_SIZE - 1) // bottom-right corner
                {
                    if (!digit_grid[i - 1][j] && !digit_grid[i][j - 1])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (i == 0) // top row
                {
                    if (!digit_grid[i][j - 1] && !digit_grid[i - 1][j] && !digit_grid[i][j + 1])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (i == GRID_SIZE - 1) // bottom row
                {
                    if (!digit_grid[i][j - 1] && !digit_grid[i + 1][j] && !digit_grid[i][j + 1])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (j == 0) // left column
                {
                    if (!digit_grid[i + 1][j] && !digit_grid[i][j + 1] && !digit_grid[i - 1][j])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                else if (j == GRID_SIZE - 1) // right column
                {
                    if (!digit_grid[i + 1][j] && !digit_grid[i][j - 1] && !digit_grid[i - 1][j])
                    {
                        digit_grid[i][j] = 0;
                    }
                }
                // for the inner side of the matrix:
                else if (!digit_grid[i + 1][j] && !digit_grid[i - 1][j] && !digit_grid[i][j + 1] && !digit_grid[i][j - 1])
                {
                    digit_grid[i][j] = 0;
                }
            }
        }
    }
}