Place pork ribs, onion, and tomatoes in the Instant Pot.
Add water and sinigang mix. Close the lid and set to high pressure for 20 minutes.
Release pressure naturally for 10 minutes, then manually release the remaining pressure.
Add daikon radish and eggplant to the pot. Close the lid and cook on high pressure for 5 minutes.
Release pressure naturally for 5 minutes, then manually release the remaining pressure.
Add water spinach and season with fish sauce to taste. Serve hot.
