If the freezer works well, there's probably a limited amount of things that can go wrong to prevent fridge from being cold.
Why isn't the air moving from freezer to fridge? You can usually feel the air moving.
You stated fan works.
Were ducts blocked by something? Ice?
This can be hard to troubleshoot once you unplug and manually defrost as problem goes away until ice builds up again.
If ice, is it there because freezer is not defrosting, or defrosting but not draining so ice reforms?
I've also seen defrost heater get stuck on.