I am from same SG and an earlier victim of BDB. Don't stop your fight. You need to escalate further. I got partial money back.
Same here, where banks initially didn't accepted my dispute i.e with same reason that transaction was authorized. Raised a case with reason "Services not rendered as described" and with all supporting documents.
Good that you approached lawyer. If still banks didn't accept dispute case, include MAS in your dispute mails to banks. Banks need to accept your case.
After escalation, at least MAS added these merchants in their fraud list. However, I am still requesting for a complete ban (same as done in US, Australia etc,.)
All the best in your fight and feel free to ping privately if require any assistance.
How did you approach the banks on this? Because there does not seem to be any forms to file or anything, but when I call them up, they straightaway tell me that I do not have a case as I authorised the transactions with a one-time pin.. then they'll just tell me there's nothing they can do for me, give me awkward silence and that's it!