Your posts say that BOTH the freezer and fridge portions are getting inadequate cooling. Further, you report the fan is working and air is flowing. So it is likely the problem is with the parts that generate cold, and not the movement of cold air. That could have two sources (or maybe more I have not considered).
1. The refrigeration loop has a small leak so that it has lost a significant amount of its coolant gas over time. If this is the cause, a good technician can test for that and re-fill the system. Not cheap, but not a huge cost. BUT the real issue is that finding and patching the leak is extremely difficult, and the problem WILL come back if it is not fixed.
2. This one is no-cost IF you consider yourself a reasonably good handyperson. I have done this several times on our older fridge. Most such fridges are the "auto-defrost" type. The actual cooling coils are in a compartment in the bottom or back of the freezer. The fan circulates air through the compartment and into the freezer, and a small portion of that air flow is diverted through the fridge portion. In normal operation some frost builds up on the freezer coils, and a sensor detects when that is too much. It triggers a timed sequence to shut off the compressor, turn on a heater near the coils, and melt that frost off. The water collected under the heated coils flows to a drain hole at the bottom rear of the chamber, down a bit of tubing and out of the back of the fridge near the top into a vertical plastic drain hose that empties into a shallow tray under the compressor at the very bottom of the fridge. After all the frost has been melted and drained to that tray, the system returns to normal cooling operations. Over the next many hours the water in the tray, warmed slightly by heat from the compressor, evaporates. The "problem" I have fixed is that the drain tubing system sometimes accumulates dust, "crud", whatever, and gets blocked somewhere so that the water cannot drain out. It gets trapped in the freezer coil chamber and simply re-freezes there, greatly reducing ability of the coils to chill the air being circulated by the fan. This leads to more frequent defrost cycles, and often to a drain pan at the bottom that never gets wet.
(Revised to include the obvious.) Unplug the fridge and remove the freezer contents to an insulated container. Start by pulling the fridge out so you gain access to the back with its black tubes and drain hose. See if you can detach that drain hose and ensure it is clear of dirt clogs. Depending on your particular system, from the front you may be able to find a way to remove the cover at the back of the freezer with the air flow slots and the temp control knob to expose the cooling coils. Be VERY careful NOT to damage those finned coils. Do NOT ever try to SCRAPE frost from them. Carefully clean out this whole area. Find the drain hole at the back bottom and try to run a flexible wire or pipe cleaner down it and out of the back where the drain hose connects, to clear out any debris in that passage. Once you have the entire drain system cleaned out, restore the freezer interior panel and knob, then reconnect the drain hose on the back and return the fridge to normal operation. Replace the freezer foods and plug in. IF this really was the cause of the poor fridge performance it should be much better now, and over the next few days you MAY observe water at times in the tray at the very bottom - IF you can even see there.