Delayed possession property transferred to an another developer

Dear SIr/ Madam, I had booked a property in May 2017 and executed an agreement of the sale for the same with a promise of getting a possession by Dec 2019. The builder failed to do so and did not lay a single floor. Nor did he respond adequately to our pleas for an alternate solution. Finally we filed a complaint in RERA in April 2020. Apart from the fact that there is still no scheduling of the hearing as of now, a new builder has now stepped in at the same site and constructing the building and selling the flats as well. We were not informed about this by the old builder or the new builder, no consent was taken and old builder says that he has directly transferred the building to the other developer and we have no claim there. The new builder has a new registration number from RERA and then old one(against which we filed the complaint) still remains in the system. Can you please tell me how to proceed in this situation?