The landlord refuses to return the advance security deposit

The rent agreement is not in force as on date. Legally, there is no way to finalise the issue.
You have to settle the matter mutually with discussions with the landlord and in this issue, you can take the help of the association.
You can talk to the landlord and tell the problems you are facing and how promptly you have paid the rent and other payments without delay and ask him to return the advance and then vacate the portion. I think the owner will accept.

The rent agreement expired in May 2020 and could not be renewed due to Covid-19 situation. You could have gone for a simple agreement on white paper duly signed by him and the tenants and also making a reference to the earlier rent contract and mentioning the new rent rate as agreed between the parties. Once that would had been prepared and witnessed by some local witnesses then that could had temporary served the purpose and he had to be legally bound by that and nothing arbitrary could had been initiated from his side. You can still talk to him and make an agreement on plain paper citing the pandemic reason. Anyway, now you have to talk to him and try to settle the score amicably. Please do not take law in your hand by not paying him rent without informing him as that would complicate the matter. One thing which you can do as the last resort is you can send a registered acknowledgement letter mentioning these things and then intimating him that he has to return or adjust the security deposit failing which you might go for legal action. In that you can also mention that if he does not comply with the terms and conditions which were earlier agreed under the agreement and now under the verbal communications then you might be forced to temporary withhold the rent payment. If you have some confusion in the matter take help of a lawyer who would guide you properly in the matter.

1. Hope that you have a copy of agreement while depositing the security amount.

2. Hope you have receipt of rent up to September 2020.

3. You have right to see all receipt of repair work got done by your landlord. He has to give you a complete list of the damaged items and the cost to repair them and the list must say that he "swears under penalties of perjury" the list is true and he must sign it.

4. You should give him a written letter to return your security amount or give a written request to the secretary/ president of society and keep the receipt.

5. If nothing works, contact a lawyer for legal action. Your landlord will return your all amount with interest and total expenses for taking this legal action as well.

6. Don't forget to take photographs or videography of your flat.

7. When you talk to him directly or through society use the hidden camera and record his statement as the evidence for legal proceedings.

As there is no valid renewed rental agreement, you cannot insist to to stay on. Actually you are now not staying there and just paying the rent. You are agreeing to vacate, but want the deposit given back. It is not given how much is the deposit.
Do you have proof of the deposit paid?
Do you have the proof of monthly rent paid till date from June 2020 and also the months previous to that?
Are there any valuable things of yours in the room? In case of legal case or mediation, these may become necessary.
You can send him a registered acknowledgement letter saying that though you are willing to vacate as per his requirement, you may state the practical difficulties because of Covid situation and related restrictions. You may say that you will be vacating by so-and-so date due to this reason. You tell (write) him that he should return the deposit at the time of vacating. You can also send by E mail and Whatsapp.
You need not worry about the flat maintenance amount, if the rent agreement (the previous one) does not specify it as your responsibility.
If there are no valuable things of yours in the room, then you can tell him to take it that you have vacated from so-and-so date and keep all your belongings packed ready for you to take back when the travel restrictions ease. From now on you need not pay rent at all.
The maximum he can do is to throw out or remove and keep elsewhere your things. You need not worry about NOC etc unless you have given some signed paper or agreement to society stating your responsibilities. Even if so as far as it is paid till May 2020 you are free.