RGBSI - Complaint on company HR

I worked as an CAE senior analyst for a company named Stellantis in chennai and my vendor company was RGBSI(bangalore).
On *rd oct **** i put my papers and served my * month notice period which is mentioned in my offer letter.
During this period from the day of my resignation there is no response from my vendor team regarding informing any details about my last date and exiting procedure even I was back of them since *th oct **** till the end of my notice period . I tried to contact them through calls and whatsapp dms but there is no response from them.
so after my notice period is done as there is no response from my vendor i had to stop working then they started approaching me.
Then i explained about my issue ,they said that once i return there accessories they will start my exit process but soon after returning there is no response from them until today.
They made me work really hard, i used to work until late nights *am from morning *am. I know the pain and mental stress i have been through .
I want my experience certificate and relieving letters for what i have worked for.