To the best of my knowledge you have to serve 90 days Notice or adjust Notice Pay accordingly. The best way is to approach the HR/HOD/Senior management team and request for waiver of the Notice, and clear the balance FnF/Relieving. Wish you all the Best.
Uncommunicated order about confirmation before submission of your resignation letter has no relevance. The company's action is wrong if your appointment order clearly specifies that confirmation will be made by issue of order. In that case the confirmation cannot be deemed to have been made before you submitted your resignation.
Your latest post, asking about legal notice and under which act you should proceed, suggests as if you are a law student, not a sufferer having any such problem in reality. Otherwise, you would have asked how to proceed further when the company has served you with a recovery letter. When you have been provided with appropriate suggestion, your first action is to get your problem sorted out administratively by replying to the recovery letter, not to think about issue of notice or to ask for the act under which you can proceed in the court of law. It is not the time for repeated queries, but to reply befittingly to the company, as against their recovery letter, when they have not communicated you with any confirmation order.
You could have thought about legal; notice only when they would have persisted with their assumption about your confirmation. Moreover had your problem been real one, what would you have done by knowing the act under which you should proceed, when the case is supposed to be fought by your lawyer? Still further, would you prefer to spend more on the lawyer's fee and other incidentals than your dues of basic salary for 34 days [54 (-) 20 days] from the company?
Though all statutory contributions and payments like ESI,PF, Bonus, leave with pay etc are available to probationers there are some benefits like advance from the company, leave travel concession, medical facilities to family members (other than that provided by ESIC) and other non statutory welfare measures, which are available only to confirmed employees. Please check.
Before filing a case against your (past)employer please understand that you will be wasting your time; obviously will have to tale leaves very frequently and you will have to reveal to your present employer that you are fighting a case against your previous employer and the reason is so and so. This information may be irritating for your present employer and may even lead to think that your probation need not be extended but put an end by giving you a termination letter before you become a confirmed employee!
As already stated, there is no written law on probation and confirmation and everything depends on the contract of employment or the company's policy. Even the period of probation may be different for two companies. Therefore, take it in good spirit and negotiate with the employer and get relieved. From the information about the salary it also happens to know that you will not get protection of Labour Laws.