Resignation during Probation period

I joined a listed company on 12th July 2019, the terms in appointment letter are as follows " Probationary Period: You will initially be on probation for the period of six months from the actual date of your joining us and would continue to be so unless and until you are expressly confirmed in the regular services of the company. During the probation period, your services are liable to be dispensed with at any time, without any notice/compensation or assigning any reasons thereof at the sole discretion of the company. Services after confirmation are terminable by one month notice for designation below the managerial level and form senior manager onwards the notice period is three months in writing by either side. During the probation period, your performance and other antecedents will be thoroughly assessed / evaluated by your superiors and only on satisfactory completion of your probation period, you will be confirmed in the regular services of the company on such terms and conditions as decided by the company" During 7th Aug 2020, the company has given me probation extension order for 6 more months which completes on 2nd Feb 2021. however, on 28th Jan 2021 I have submitted my resignation letter via my official Mail ID, there is no response to that mail even after followup mails. In what's up my manager says that "during probation period your services are liable to be dispensed with at sole discretion of management. it does not mention from either side" and asking me to serve the notice period Kindly advice way forward