1. If your last date of working with the Company is 30.7.2021 after serving the due notice as per your employment contract, then you should write to your HR about your inability to continue any further since you have resigned following the rule of the Company. However, make sure that you have resigned giving notice for the period as specified in your employment terms.
2. I shall always advise to avoid any direct confrontation with your employer because our law is not employee friendly.
3. Try to settle the matter amicably with your present Company or negotiate with your next Company for extending your joining time or allowing you to join them without any release or experience letter, in case they do not want to extend your joining time.