Imagine that the requirement was "or" instead. Then m = 2 corresponds with n = 2. This is because the subdiagonal is always homogeneous (there is only 1 subdiagonal entry).
You can partition this matrix into three pieces. The first piece is an $n \times n$ submatrix in the bottom left corner. The second is an $n \times n$ piece in the top right, and the third is a thin strip of a single column and single row.
Now by the induction hypothesis, you can transform the bottom left into a $m \times m$ matrix with the property without affecting the top right corner whatsoever.
Anyways, you'll have two $m \times m$ matrices in this reduced form touching diagonally. From there, you take a few simple cases and you can always be guaranteed an $m+1 \times m+1$ matrix with diagonal entries equal.
It's pretty hard to explain without a blackboard unfortunately, and I have no motivation to draw MSpaint pictures since I was interpreting the problem incorrectly, lol