Resignation

Sir I am working as Asst Professor in an organization from 2016 to till the date My organization is not giving me Employee Provident fund and this year some new faculties joined for them they are giving PF for me while joining orally they said we will give PF from day 1 but after one month our principal said no PF for you its management decision. I accepted it and worked sincerely till today and recently our order letter is updated and they have mentioned if you all not meet the expected performance Continuation of your job is up to the management and also they interfere in our research (Phd ) they forced us to drop the research in the middle ,for our research management not sponsored single rupee. and in my updated offer(order) letter they did not mention any notice period . I resigned on April 19 2022 now they are not ready to relieve me they mentally harassing me. for me they forced to change my research supervisor in the end stage during covid time they harrash me lot . I need my experience letter for next job.any legal solution please suggest me .how to handle this