I have told to the support I use a VPN but he don't care.
I think that was the reason why have detected you using multiple account. Most likely they have checked your IP address, and usually it would happened if you had used VPN. Perhaps some other users used same VPN or IP address that you had used. Is it against their terms & conditions? I am not sure about it since I had not read. I don't think they will scam this kind of small amount since they have solve more issue with more big amount. Try to discuss their ANN thread as coinlocket$ already mentioned, hope your problem will solve.
The VPN seems not to be issue here base on the OP statement about getting inform that he used VPN and VPN was not what the sportbet claimed was the reason why the OP fund are withheld but it will be better if the OP could right but I will advice the OP to PM and complaint on their official ANN thread as said earlier. If no change, take it to ask gambler.