Experience letter

Hello Experts I have worked in an organization for 2 years and have all supporting documents. I submitted my resignation in Feb and according to employment terms I served my notice period till apr 2018 and handed over all my office belongings to the organization. While leaving they said that relieving letter and experience certificate will be sent to me with in few days. Now after more then a month when I asked for the same documents they say that my resignation wasn't excepted and I can't join any other organization because it will be treated as dual employment and they mention that we allow you for leave of absence for the period you are willing for. Now I do not have any written mail for resignation approval neither do they sent me any mail for non acceptance or any mail or written request for me asking for any sort of leave. My resignation was approved in a face to face meeting and I never bothered of acceptance in mail. To join next organization I need either experience or relieving letter. What options do I have? I have offer of employment now and after 15 days of joining I need to show clear exit from previous organization. What are my options?