My previous employer doesn’t provide experience certificate.

Hi! After graduation I have worked in a small company in Surat GIDC. They used to give salary in cash. No salary slip provided. No PF also given. So I left the company in 4 months. They have not given me experience letter. . After that I worked in a small company in Mumbai. They use to give salary in bank account. Initially there was no PF. After almost 1 year of joining they started PF. But employers share also deducted from our salary. So in hand salary was very low. They always delay salary. Therefore out of frustration I gave resignation. But at a time of giving resignation they extended my notice period to 2 months. I was unable to serve the notice period. So they did not give me resignation approval letter, reliving letter and experience letter. . After 6 months of unemployment I got new job in L&T by third party payroll. At the time of interview I explained L&T interview panel my previous employment situation. They are okay with it. And selected me also. Through third party vendor I received offer letter. Third party vendor asking me all experience letter, resignation approval letter. Without that third party vendor will not give me appointment letter. But I am working in L&T from 24 Feb 2023. Till now I have not received appointment letter. What shall I do?