Can I touch the net or reach under or over it?

Yes, within limits.

• Contact with the net by a player is not a fault unless it interferes with play.
• A player may touch the post, ropes or any other object outside the antennae, including the net itself, provided it does not interfere with play.
• It is permitted to penetrate into the opponent's space under the net, and to touch the opponent's court beyond the centre line with any part of the body, provided it does not interfere with the opponent's play.
• A player may enter the opponent's court after the ball goes out of play.
• In blocking, a player may touch the ball beyond the net, provided he or she does not interfere with the opponents' play before or during their attack hit.

Source: World ParaVolley Official Sitting Volleyball Rules 2025-2028, Rules 11.1 to 11.3.