You need to determine if you were performing a "contract of service" (ie employment) or "contract for service". Did this company contribute to
CPF? What about IR8?
You need to determine the precise terms that bind you and this company.
Why don't you see a lawyer? It will cost you approx SGD 300 for a consultation but you will know for sure your position. If need be, you can then send a demand letter to this company.
All the best.