i want the fixed rows border to be solid while the rest dotted
There's no easy way to do this.
You might be able to do it with a DrawFilter, but it's a bit tricky.A Cell or Row does not draw all 4 of it's own border sides, because if it did, you would end up with double-thick borders between every cell. So you would have to determine not just if the row is fixed, but also if it is the last fixed row and which borders each cell is drawing.