Place first eight ingredients (through pepper) and 1/4 tsp of the salt in a 6-qt Instant Pot. Place wire rack in the pot. Add potatoes on top of rack. Place frozen fish on top of potatoes. Sprinkle remaining 1/4 tsp salt on fish. Lock in place. Set on High Pressure to cook 10 Minutes. Release pressure quickly. Open lid carefully.
Use wire rack to lift potatoes and fish from cooker. Divide broth mixture and potatoes among 4 serving bowls and top with fish. Place 1/2 Tbsp butter on top of each fish portion. Sprinkle with herbs and serve with lemon wedges.