Preheat your air fryer to 400°F (200°C).
Slice the Russet potato into thin wedges, then toss them in a bowl with half of the olive oil and a pinch of sea salt.
Place the potato wedges in the air fryer basket in a single layer and cook for 15-20 minutes, shaking halfway through, until they are golden and crisp.
In a small mixing bowl, whisk together the arrowroot powder, light beer, garlic powder, paprika, remaining sea salt, and black pepper until a smooth, thin batter forms.
Pat the cod fillet dry with a paper towel, brush with the remaining olive oil, and then dip the fish into the beer batter until it is fully coated.
Place the battered fish into the air fryer basket alongside the potatoes (or in a second batch) and cook for 10-12 minutes at 400°F until the coating is crunchy and the fish flakes easily with a fork.