There is a very clear pattern in the data as shown here . Your 20 values are adequately descRibed by a hybrid model using an AR(1) and a step/level shift along with 3 pulses .
and here
and here
The tools (approaches) that you were considering are presumptive in form whereas your data has it's own message. This data has not only a strong memory but has been affected by external activity causing the step.level shift and the 3 pulses.
The method used here to form the model is called Intervention Detection . Search SE for "INTERVENTION DETECTION"