Non transfer of car ownership by used car dealer after car purchase

I have bought a used car from a used car dealer on the month of August 2020. After checking and verification of the ownership online through parivahan website, found the RC details matching the government details. After checking the details i purchased the vehicle. owner transfer papers and rto forms were duly signed by me and the sign of the first owner was already there on those documents. since then the dealer is not transferring the ownership of the vehicle to my name by giving some or the other reasons. Now he has also stopped receiving my calls. how should i proceed further with this to get the ownership of the vehicle transferred to my name. Should i file a FIR against him or should i send him a legal notice marking the 1st owner as i have his details through the RC