jimmy, I don't have an air fryer but I was thinking if I were to try to make porridge in the toaster oven on convection (almost the same thing).
I rather think it would take a rather long time to bake. I don't know your method for making porridge but mine is to add ingredients to water in a pot, bring to boil (on stove top) and let simmer til done. To achieve that in an oven, if you were to boil the water first (assuming you have an electric kettle), mix hot water into your porridge bowl, then bake at maybe something like 400f ...
have no idea how long it would take, but as long as it is sort of bubbling, without overflowing.
Hope someone with a bit more know how comes along to help you!
Are you referring to oatmeal or congee? For either, if you are working with limited space and equipment, you might want to invest in an Instant Pot. You can make either in that, plus a multitude of braises and other dishes.