Perhaps you were using a VPN?

There might be issues with that because a VPN reuses the same IP for different users and one or more of them may have also registered accounts at Rollbit. But they should not be allowed to withhold your deposited funds, only the funds you have won.

Casinos have a strict no multi-accounting rule because people tend to abuse the bonus systems and so on.

If you have not used or registered any alternative accounts than state this fact to them.