As you told that only offer letter was given to you at the time of joining. So confirm these things:
Firstly: Whether the rules regarding resignation were mentioned on that letter or not.
Secondly: Whether the terms of employment mentioned such notice period were provided.
If either of these were not provided then
you should give a legal notice to them regarding payment as per the contract between you and company, you are entitled to receive such payment.
In default of payment you can file suit in the area where the company is situated under Specific Relief Act.