The fridge came with a house I bought and is/was in nice condition, and looks fairly new. Has worked great for 2+ years until the last couple months.

It seemed it might not be as cold or just not turning on as much so I upped the thermostat a little bit to "colder". In the last month the freezer has actually been getting warm enough to put frost/thaw food a little bit. Today I woke up and the freezer was completely unfrozen and fridge was warm too.

But as I type this it just turned on again? I have the thermostat all the way up to "coldest" because that seems to be the only way it will even turn on.

So,

-Went from working great
-Not staying as cold/not turning on as much, so I turned thermostat up a little. Not all the way, maybe 3/4 or a bit less. Normal was middle.
-Freezer/Fridge gets almost warm now.
-Only way it will turn on if the thermostat is all the way to "coldest" or I would have to move it there and move it back

Dead? Any ideas?

Thanks, Can't really afford a new one, might just have to use the garage for a while if it stays cold.

Here are your parts
Replacement parts for WHIRLPOOL RT21LMXKQ03 REFRIGERATOR | AppliancePartsPros.com

Since the unit is cooling OK when the compressor is on and you can get it to go on by rotating the control, I would say that the problem is the thermostat itself (Item 13 in Section 3).

They can go where they stick open or closed and this can also be an intermittent.