Start with the "burner area tune-up" as described in your refrigerator owners manual (and many other places).
It doesn't take much debris in the burner tube to make the flame too small to do an adequate cooling job.
Also, it takes 24 hours for both freezer and refrigerator to come to temperature on an absorption refrigerator. Has it been long enough?
Sure, it could be a bad cooling unit, but start with the easy (no parts and no special tools needed) stuff.