Using this file for inspiration:

https://github.com/MrCrayfish/MrCrayfishFurnitureMod/blob/1.15.X/src/main/resources/data/cfm/recipes/cooked_potato_from_grill_cooking.json

I created a datapack with the following recipe for the Aquaculture fish fillet:

{
    "type": "cfm:grill_cooking",
    "ingredient": {
        "item": "aquaculture:fish_fillet_raw"
    },
    "result": {
        "item": "aquaculture:fish_fillet_cooked"
    },
    "experience": 4.0,
    "cookingtime": 600
}

But it doesn't appear to have done anything. Am I missing something?

After testing, it's seems it's working as normal.
Test Pack: Test.zip