Owner is not giving our refund back and not agreeing to notice period

Hi All, Background: Started living in flat in Oct 2018, signed a 11 month non-notarised rental agreement. Did not receive any rent agreement after that. Gave notice period in October 2020. My friends and I informed our owner on 27th September 2020 over a call, that we will be vacating the flat. We also texted the same to him over Whatsapp on 5th October 2020. But when we called him around 15th October, he said the communication was not proper i.e. over email/written letter and he will consider the notice period of two months from next month, ie starting from 1st Nov 2020 and ending on 31st Dec 2020. 1. We had no issues with the owner and vice-versa during our 2 years' stay. We always paid rent on time. Both parties used to mutually communicate over Whatsapp and all. 2. The owner did not give us the revised rental agreement in 2019 and 2020. We reminded him for the same multiple times. But we timely paid him the 10% increased rent after September 2019 and again 10% increased rent after August 2020. 3. It is clearly mentioned in the only rent agreement (2018 one), "The lessee can vacate the premises after giving TWO months' notice to the lessor". It is nowhere mentioned that notice period will start from 1st of the next month. Ideally, we informed him on 5th October, but we agreed to his terms, that notice period was given on 15th October 2020. After several emails, he agreed over the email that we can serve 15th Oct-15th Dec notice period. 4. Now, out of the blue, after our last communication on 27th October, he is emailing us in December that "I got a reformation from my lawyer. So the issue is that two notices cannot co-exist i.e. the monthly advance rent becomes due on 1st of each month. So even if the notice for vacation is given after 1st then the notice of vacancy can start from next month only. So you have to serve till 31st Dec 2020" Now, he's emailing us telling about lawyers and all, so I wanted to ask opinion from you all respected lawyers. a) How valid his assumption is? 2months should ideally start from the day we gave vacancy notice. It is not mentioned in the old agreement also. b) We did not want any problems with this, we wanted to settle on a middle ground. But suddenly he is coming with new statements that we have to serve till Dec 31st. We could have disagreed with him on increased rents/notice period since there was no rent agreement signed by us, but we never did, as we wanted a good equation with him. So, what should we do now? Should we file a complaint? Should we serve notice period till 15th dec? Till 31st Dec? What if he refuses to give the security back?